diff --git a/app/renderer/App.tsx b/app/renderer/App.tsx
new file mode 100644
index 0000000..84f3256
--- /dev/null
+++ b/app/renderer/App.tsx
@@ -0,0 +1,153 @@
+import { useState } from 'react'
+
+import { Hint } from './components/Hint'
+import { Panel } from './components/Panel'
+import { Sidebar, type Section } from './components/Sidebar'
+import { Stat } from './components/Stat'
+import { TopBar } from './components/TopBar'
+import { Window } from './components/Window'
+import { usePolled } from './hooks/usePolled'
+import { codeburn } from './lib/ipc'
+import type { MenubarPayload, Period } from './lib/types'
+
+const SECTION_TITLES: Record<Section, string> = {
+  overview: 'Overview',
+  spend: 'Spend',
+  optimize: 'Optimize',
+  models: 'Models',
+  plans: 'Plans',
+  settings: 'Settings',
+}
+
+const PERIOD_LABELS: Record<Period, string> = {
+  today: 'Today',
+  week: 'Last 7 days',
+  month: 'This month',
+  '30days': 'Last 30 days',
+  all: 'All time',
+}
+
+const STANDARD_PERIODS: Period[] = ['today', 'week', '30days', 'month', 'all']
+
+function isPeriod(value: string): value is Period {
+  return (STANDARD_PERIODS as string[]).includes(value)
+}
+
+function fmtUsd(n: number): string {
+  return `$${n.toLocaleString('en-US', { minimumFractionDigits: 2, maximumFractionDigits: 2 })}`
+}
+
+export function App() {
+  const [section, setSection] = useState<Section>('overview')
+  const [period, setPeriod] = useState<Period>('30days')
+  const provider = 'all'
+
+  // Polled at the app level: proves the spawn→IPC→render path and feeds the
+  // sidebar status line. Section-specific fetching lands in T2–T7.
+  const overview = usePolled<MenubarPayload>(() => codeburn.getOverview(period, provider), [period, provider])
+
+  const onPeriodChange = (value: string) => {
+    // 6M / Custom (date ranges) are not wired until T8; ignore for now so the
+    // highlight never lies about what was fetched.
+    if (isPeriod(value)) setPeriod(value)
+  }
+
+  const scope = `${PERIOD_LABELS[period]} · All providers`
+
+  return (
+    <Window>
+      <Sidebar active={section} onNavigate={setSection} status={<StatusLine polled={overview} />} />
+      <div className="ct">
+        <TopBar
+          title={SECTION_TITLES[section]}
+          scope={section === 'settings' ? undefined : scope}
+          period={period}
+          onPeriodChange={onPeriodChange}
+          providerLabel="All providers"
+        />
+        <div className="body">
+          {section === 'overview' ? (
+            <OverviewPlaceholder polled={overview} />
+          ) : (
+            <SectionPlaceholder title={SECTION_TITLES[section]} />
+          )}
+        </div>
+        <Hint
+          items={[
+            { k: '⌘K', label: 'Command' },
+            { k: '⌘E', label: 'Export view' },
+          ]}
+          right={overview.loading ? 'refreshing…' : 'auto-refresh 30s'}
+        />
+      </div>
+    </Window>
+  )
+}
+
+function StatusLine({ polled }: { polled: ReturnType<typeof usePolled<MenubarPayload>> }) {
+  if (polled.data) {
+    return (
+      <>
+        {polled.data.current.label} <b>{fmtUsd(polled.data.current.cost)}</b>
+      </>
+    )
+  }
+  if (polled.error?.kind === 'not-found') return <>CLI not found</>
+  if (polled.loading) return <>scanning…</>
+  return <>—</>
+}
+
+/**
+ * Task 0 smoke view: renders the raw `current.cost` from getOverview to prove
+ * the end-to-end path. The real Overview (stat cards, capsule chart, sessions)
+ * is Task 2. CLI-missing shows an honest first-run state, never a crash.
+ */
+function OverviewPlaceholder({ polled }: { polled: ReturnType<typeof usePolled<MenubarPayload>> }) {
+  if (polled.error?.kind === 'not-found') {
+    return (
+      <Panel title="Locate the codeburn CLI">
+        <p style={{ color: 'var(--t2)', margin: '0 0 6px', fontSize: 12.5 }}>
+          CodeBurn Desktop reads your usage by running the <code style={{ fontFamily: 'var(--mono)', color: 'var(--lav)' }}>codeburn</code> command,
+          but it isn&apos;t on your PATH yet.
+        </p>
+        <p style={{ color: 'var(--t3)', margin: 0, fontSize: 11.5 }}>
+          Install it with <code style={{ fontFamily: 'var(--mono)', color: 'var(--lav)' }}>npm i -g codeburn</code>, then reopen this window.
+        </p>
+      </Panel>
+    )
+  }
+
+  if (polled.error) {
+    return (
+      <Panel title="Couldn't read usage">
+        <p style={{ color: 'var(--red)', margin: 0, fontSize: 12 }}>{polled.error.message}</p>
+      </Panel>
+    )
+  }
+
+  if (!polled.data) {
+    return (
+      <Panel title="Overview">
+        <p style={{ color: 'var(--t3)', margin: 0, fontSize: 12 }}>Scanning sessions…</p>
+      </Panel>
+    )
+  }
+
+  const current = polled.data.current
+  return (
+    <div className="stats" style={{ gridTemplateColumns: 'repeat(2, 1fr)' }}>
+      <Stat label={current.label} value={fmtUsd(current.cost)} delta={`${current.sessions} sessions`} tone="info" />
+      <Stat label="Calls" value={current.calls.toLocaleString('en-US')} delta="this period" />
+    </div>
+  )
+}
+
+function SectionPlaceholder({ title }: { title: string }) {
+  return (
+    <Panel title={title}>
+      <p style={{ color: 'var(--t3)', margin: 0, fontSize: 12 }}>
+        {title} lands in a later task. The shell, data bridge, and design system are in place.
+      </p>
+    </Panel>
+  )
+}
diff --git a/app/renderer/components/Hint.tsx b/app/renderer/components/Hint.tsx
new file mode 100644
index 0000000..28bc049
--- /dev/null
+++ b/app/renderer/components/Hint.tsx
@@ -0,0 +1,18 @@
+import type { ReactNode } from 'react'
+
+export type HintItem = { k?: string; label: ReactNode }
+
+/** The `.hint` footer strip: keycap hints on the left, optional right-aligned note. */
+export function Hint({ items, right }: { items: HintItem[]; right?: ReactNode }) {
+  return (
+    <div className="hint">
+      {items.map((item, i) => (
+        <span key={i}>
+          {item.k && <span className="k">{item.k}</span>}
+          {item.label}
+        </span>
+      ))}
+      {right !== undefined && <span className="r">{right}</span>}
+    </div>
+  )
+}
diff --git a/app/renderer/components/Panel.tsx b/app/renderer/components/Panel.tsx
new file mode 100644
index 0000000..f000d8c
--- /dev/null
+++ b/app/renderer/components/Panel.tsx
@@ -0,0 +1,30 @@
+import type { CSSProperties, ReactNode } from 'react'
+
+/** Dash0-style card: optional `.phead` title strip + `.pbody` content. */
+export function Panel({
+  title,
+  right,
+  className,
+  bodyStyle,
+  children,
+}: {
+  title?: ReactNode
+  right?: ReactNode
+  className?: string
+  bodyStyle?: CSSProperties
+  children?: ReactNode
+}) {
+  return (
+    <div className={className ? `panel ${className}` : 'panel'}>
+      {title !== undefined && (
+        <div className="phead">
+          <b>{title}</b>
+          {right !== undefined && <span className="r">{right}</span>}
+        </div>
+      )}
+      <div className="pbody" style={bodyStyle}>
+        {children}
+      </div>
+    </div>
+  )
+}
diff --git a/app/renderer/components/ProviderPop.tsx b/app/renderer/components/ProviderPop.tsx
new file mode 100644
index 0000000..789d8f5
--- /dev/null
+++ b/app/renderer/components/ProviderPop.tsx
@@ -0,0 +1,8 @@
+/** The `.pop` provider selector. Mutation (opening the menu) lands in T8. */
+export function ProviderPop({ label, onClick }: { label: string; onClick?: () => void }) {
+  return (
+    <div className="pop" role="button" tabIndex={0} onClick={onClick}>
+      {label}
+    </div>
+  )
+}
diff --git a/app/renderer/components/SegTabs.tsx b/app/renderer/components/SegTabs.tsx
new file mode 100644
index 0000000..affbf4e
--- /dev/null
+++ b/app/renderer/components/SegTabs.tsx
@@ -0,0 +1,33 @@
+export type SegOption = { value: string; label: string }
+
+/** The `.seg` segmented control used for period and lens switching. */
+export function SegTabs({
+  options,
+  value,
+  onChange,
+  style,
+}: {
+  options: SegOption[]
+  value: string
+  onChange: (value: string) => void
+  style?: React.CSSProperties
+}) {
+  return (
+    <div className="seg" style={style}>
+      {options.map(opt => (
+        <span
+          key={opt.value}
+          className={opt.value === value ? 'on' : undefined}
+          role="button"
+          tabIndex={0}
+          onClick={() => onChange(opt.value)}
+          onKeyDown={e => {
+            if (e.key === 'Enter' || e.key === ' ') onChange(opt.value)
+          }}
+        >
+          {opt.label}
+        </span>
+      ))}
+    </div>
+  )
+}
diff --git a/app/renderer/components/Sidebar.test.tsx b/app/renderer/components/Sidebar.test.tsx
new file mode 100644
index 0000000..794ca13
--- /dev/null
+++ b/app/renderer/components/Sidebar.test.tsx
@@ -0,0 +1,28 @@
+// @vitest-environment jsdom
+import { describe, it, expect, vi } from 'vitest'
+import { render, screen, fireEvent } from '@testing-library/react'
+
+import { Sidebar } from './Sidebar'
+
+describe('Sidebar', () => {
+  it('renders all six nav items', () => {
+    render(<Sidebar active="overview" onNavigate={() => {}} />)
+    expect(screen.getAllByRole('button')).toHaveLength(6)
+    for (const label of ['Overview', 'Spend', 'Optimize', 'Models', 'Plans', 'Settings']) {
+      expect(screen.getByRole('button', { name: new RegExp(label) })).toBeInTheDocument()
+    }
+  })
+
+  it('calls onNavigate with the section id when a nav item is clicked', () => {
+    const onNavigate = vi.fn()
+    render(<Sidebar active="overview" onNavigate={onNavigate} />)
+    fireEvent.click(screen.getByRole('button', { name: /Spend/ }))
+    expect(onNavigate).toHaveBeenCalledWith('spend')
+  })
+
+  it('marks the active item with the "on" class', () => {
+    render(<Sidebar active="models" onNavigate={() => {}} />)
+    expect(screen.getByRole('button', { name: /Models/ })).toHaveClass('on')
+    expect(screen.getByRole('button', { name: /Overview/ })).not.toHaveClass('on')
+  })
+})
diff --git a/app/renderer/components/Sidebar.tsx b/app/renderer/components/Sidebar.tsx
new file mode 100644
index 0000000..48c92e2
--- /dev/null
+++ b/app/renderer/components/Sidebar.tsx
@@ -0,0 +1,46 @@
+import type { ReactNode } from 'react'
+
+export type Section = 'overview' | 'spend' | 'optimize' | 'models' | 'plans' | 'settings'
+
+export const NAV_ITEMS: Array<{ id: Section; label: string; key: string }> = [
+  { id: 'overview', label: 'Overview', key: '⌘1' },
+  { id: 'spend', label: 'Spend', key: '⌘2' },
+  { id: 'optimize', label: 'Optimize', key: '⌘3' },
+  { id: 'models', label: 'Models', key: '⌘4' },
+  { id: 'plans', label: 'Plans', key: '⌘5' },
+  { id: 'settings', label: 'Settings', key: '⌘,' },
+]
+
+export function Sidebar({
+  active,
+  onNavigate,
+  status,
+}: {
+  active: Section
+  onNavigate: (section: Section) => void
+  status?: ReactNode
+}) {
+  return (
+    <nav className="sb">
+      <div className="lights"><i /><i /><i /></div>
+      <div className="app"><b>CodeBurn</b></div>
+      {NAV_ITEMS.map(item => (
+        <div
+          key={item.id}
+          className={item.id === active ? 'ni on' : 'ni'}
+          role="button"
+          tabIndex={0}
+          onClick={() => onNavigate(item.id)}
+          onKeyDown={e => {
+            if (e.key === 'Enter' || e.key === ' ') onNavigate(item.id)
+          }}
+        >
+          {item.label}
+          <span className="k">{item.key}</span>
+        </div>
+      ))}
+      <div className="push" />
+      <div className="status">{status}</div>
+    </nav>
+  )
+}
diff --git a/app/renderer/components/Stat.tsx b/app/renderer/components/Stat.tsx
new file mode 100644
index 0000000..687ee49
--- /dev/null
+++ b/app/renderer/components/Stat.tsx
@@ -0,0 +1,26 @@
+import type { ReactNode } from 'react'
+
+export type StatTone = 'info' | 'bad' | 'hot' | 'ok'
+
+/** A `.panel.stat` metric card: label strip + big value + tinted delta line. */
+export function Stat({
+  label,
+  value,
+  delta,
+  tone,
+}: {
+  label: ReactNode
+  value: ReactNode
+  delta?: ReactNode
+  tone?: StatTone
+}) {
+  return (
+    <div className="panel stat">
+      <div className="phead"><b>{label}</b></div>
+      <div className="pbody">
+        <div className="v">{value}</div>
+        {delta !== undefined && <div className={tone ? `d ${tone}` : 'd'}>{delta}</div>}
+      </div>
+    </div>
+  )
+}
diff --git a/app/renderer/components/TopBar.tsx b/app/renderer/components/TopBar.tsx
new file mode 100644
index 0000000..b8b0628
--- /dev/null
+++ b/app/renderer/components/TopBar.tsx
@@ -0,0 +1,41 @@
+import type { ReactNode } from 'react'
+
+import { ProviderPop } from './ProviderPop'
+import { SegTabs, type SegOption } from './SegTabs'
+
+/** Full period vocabulary per the wireframe. 6M/Custom (date ranges) land in T8. */
+export const PERIOD_OPTIONS: SegOption[] = [
+  { value: 'today', label: 'Today' },
+  { value: 'week', label: '7D' },
+  { value: '30days', label: '30D' },
+  { value: 'month', label: 'Month' },
+  { value: '6m', label: '6M' },
+  { value: 'custom', label: 'Custom' },
+]
+
+/** The `.bar` top bar: title, scope caption, period SegTabs, provider ProviderPop. */
+export function TopBar({
+  title,
+  scope,
+  period,
+  onPeriodChange,
+  providerLabel,
+  onProviderClick,
+}: {
+  title: ReactNode
+  scope?: ReactNode
+  period: string
+  onPeriodChange: (value: string) => void
+  providerLabel: string
+  onProviderClick?: () => void
+}) {
+  return (
+    <div className="bar">
+      <div className="t">{title}</div>
+      {scope !== undefined && <span className="scope">{scope}</span>}
+      <div className="sp" />
+      <SegTabs options={PERIOD_OPTIONS} value={period} onChange={onPeriodChange} />
+      <ProviderPop label={providerLabel} onClick={onProviderClick} />
+    </div>
+  )
+}
diff --git a/app/renderer/components/Window.tsx b/app/renderer/components/Window.tsx
new file mode 100644
index 0000000..65373c3
--- /dev/null
+++ b/app/renderer/components/Window.tsx
@@ -0,0 +1,6 @@
+import type { ReactNode } from 'react'
+
+/** The `.win` shell: sidebar + content area live inside as children. */
+export function Window({ children }: { children: ReactNode }) {
+  return <div className="win">{children}</div>
+}
diff --git a/app/renderer/hooks/usePolled.ts b/app/renderer/hooks/usePolled.ts
new file mode 100644
index 0000000..a30611a
--- /dev/null
+++ b/app/renderer/hooks/usePolled.ts
@@ -0,0 +1,60 @@
+import { useCallback, useEffect, useState } from 'react'
+
+import { normalizeCliError } from '../lib/ipc'
+import type { CliError } from '../lib/types'
+
+export type Polled<T> = {
+  data: T | null
+  error: CliError | null
+  loading: boolean
+  /** Re-run the fetcher immediately (period/provider change, manual refresh). */
+  refresh: () => void
+}
+
+/**
+ * Generic CLI-backed data hook: fetches on mount + whenever `deps` change, then
+ * re-polls every `intervalMs`. Errors are normalized to the CliError shape so
+ * sections can branch on `error.kind`. Last-good data is retained on error.
+ */
+export function usePolled<T>(fetcher: () => Promise<T>, deps: unknown[], intervalMs = 30_000): Polled<T> {
+  const [data, setData] = useState<T | null>(null)
+  const [error, setError] = useState<CliError | null>(null)
+  const [loading, setLoading] = useState(true)
+
+  const load = useCallback(() => {
+    let cancelled = false
+    setLoading(true)
+    fetcher()
+      .then(result => {
+        if (cancelled) return
+        setData(result)
+        setError(null)
+      })
+      .catch(err => {
+        if (!cancelled) setError(normalizeCliError(err))
+      })
+      .finally(() => {
+        if (!cancelled) setLoading(false)
+      })
+    return () => {
+      cancelled = true
+    }
+    // deps are intentionally the caller-provided dependency list.
+    // eslint-disable-next-line react-hooks/exhaustive-deps
+  }, deps)
+
+  useEffect(() => {
+    const cancel = load()
+    const id = setInterval(() => load(), intervalMs)
+    return () => {
+      cancel()
+      clearInterval(id)
+    }
+  }, [load, intervalMs])
+
+  const refresh = useCallback(() => {
+    load()
+  }, [load])
+
+  return { data, error, loading, refresh }
+}
diff --git a/app/renderer/index.html b/app/renderer/index.html
new file mode 100644
index 0000000..91c69f9
--- /dev/null
+++ b/app/renderer/index.html
@@ -0,0 +1,34 @@
+<!doctype html>
+<html lang="en">
+  <head>
+    <meta charset="UTF-8" />
+    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
+    <meta
+      http-equiv="Content-Security-Policy"
+      content="default-src 'self'; script-src 'self' 'unsafe-inline'; style-src 'self' 'unsafe-inline'; img-src 'self' data:; font-src 'self'; connect-src 'self' ws: http://localhost:5173"
+    />
+    <title>CodeBurn</title>
+    <style>
+      /* App-shell glue only — indigo.css is the verbatim wireframe port.
+         Make the window fill the OS window instead of the doc-page card. */
+      html,
+      body,
+      #root {
+        height: 100%;
+        margin: 0;
+      }
+      #root {
+        display: flex;
+      }
+      .win {
+        flex: 1;
+        min-width: 0;
+        border-radius: 0;
+      }
+    </style>
+  </head>
+  <body>
+    <div id="root"></div>
+    <script type="module" src="./main.tsx"></script>
+  </body>
+</html>
diff --git a/app/renderer/main.tsx b/app/renderer/main.tsx
new file mode 100644
index 0000000..6d4d64d
--- /dev/null
+++ b/app/renderer/main.tsx
@@ -0,0 +1,14 @@
+import { StrictMode } from 'react'
+import { createRoot } from 'react-dom/client'
+
+import { App } from './App'
+import './styles/indigo.css'
+
+const root = document.getElementById('root')
+if (!root) throw new Error('#root not found')
+
+createRoot(root).render(
+  <StrictMode>
+    <App />
+  </StrictMode>,
+)