fix(date-range): unify 'all' period semantics between CLI and dashboard

`getDateRange` was duplicated across `src/cli.ts` and `src/dashboard.tsx`
with conflicting semantics for `'all'`. The CLI intentionally bounded
`'all'` to the last 6 months (justified inline: keeps Codex/Cursor parses
responsive on sparse multi-year history). The dashboard returned
`new Date(0)` instead, so the same `--period all` flag silently meant
two different windows depending on which entry point you hit.

`Period`, `PERIODS`, `PERIOD_LABELS`, and `toPeriod` were duplicated as
well, and `cli-date.ts` already existed for date helpers
(`parseDateRangeFlags`) so the consolidation lives there.

Both call sites now go through a single `getDateRange(period: string)`
in `cli-date.ts` that returns `{ range, label }`. The dashboard wraps it
as `getPeriodRange(period: Period)` to keep the strict `Period` type at
the React boundary while letting the CLI continue to accept extras like
`'yesterday'`.

`PERIOD_LABELS.all` becomes `'6 Months'` (short, for the dashboard tab
strip; the previous `'All Time'` was misleading and the long-form
`'Last 6 months'` from `getDateRange().label` already drives CLI output).

Changes:
- src/cli-date.ts: add `Period`, `PERIODS`, `PERIOD_LABELS`, `toPeriod`,
  `getDateRange`. Pull the existing 6-month rationale into a named
  `ALL_TIME_MONTHS` constant.
- src/cli.ts: drop the local copies and import from cli-date.
- src/dashboard.tsx: drop the local copies, route through
  `getPeriodRange`, alias the shared `getDateRange` import to
  `getDateRangeShared` to avoid shadowing the wrapper.
- tests/cli-date.test.ts: 13 cases covering `'all'` regression guard
  (must never silently fall back to `Date(0)`), CLI/dashboard agreement,
  end-of-month clamping tolerance, `'yesterday'` support, and
  unknown-input fallback.
- README.md, CHANGELOG.md: surface the bound and point heavy users at
  `--from`/`--to` for unbounded windows.

The CLI flag `--period all` continues to be accepted; only the dashboard
window changes to match what the CLI was already doing. No public API
or schema change.

Refs #93

src/cli-date.ts

@@ -1,4 +1,5 @@
 import type { DateRange } from './types.js'
+import { toDateString } from './daily-cache.js'
 
 const ISO_DATE_RE = /^\d{4}-\d{2}-\d{2}$/
 
@@ -7,6 +8,35 @@ const END_OF_DAY_MINUTES = 59
 const END_OF_DAY_SECONDS = 59
 const END_OF_DAY_MS = 999
 
+// "All Time" is intentionally bounded to the last 6 months. Older data is
+// rarely actionable for a cost tracker, and capping the range keeps the parse
+// path bounded so providers like Codex/Cursor with sparse multi-year history
+// still load in seconds. Users who need an unbounded window can use
+// `--from` / `--to`.
+const ALL_TIME_MONTHS = 6
+
+export type Period = 'today' | 'week' | '30days' | 'month' | 'all'
+
+export const PERIODS: Period[] = ['today', 'week', '30days', 'month', 'all']
+
+// Short labels suitable for the dashboard tab strip. Long-form labels for
+// header text come from `getDateRange().label`.
+export const PERIOD_LABELS: Record<Period, string> = {
+  today: 'Today',
+  week: '7 Days',
+  '30days': '30 Days',
+  month: 'This Month',
+  all: '6 Months',
+}
+
+export function toPeriod(s: string): Period {
+  if (s === 'today') return 'today'
+  if (s === 'month') return 'month'
+  if (s === '30days') return '30days'
+  if (s === 'all') return 'all'
+  return 'week'
+}
+
 function parseLocalDate(s: string): Date {
   if (!ISO_DATE_RE.test(s)) {
     throw new Error(`Invalid date format "${s}": expected YYYY-MM-DD`)
@@ -37,3 +67,58 @@ export function parseDateRangeFlags(from: string | undefined, to: string | undef
   }
   return { start, end }
 }
+
+/**
+ * Returns the date range and a human-readable label for a named period.
+ *
+ * Accepts a string (rather than the strict `Period` type) because the CLI
+ * surfaces a few extra inputs not exposed in the dashboard tab strip
+ * (e.g. `'yesterday'`). Unknown values fall back to `'week'`.
+ *
+ * Note: `'all'` is bounded to the last 6 months. Use `--from`/`--to` for
+ * an unbounded historical window.
+ */
+export function getDateRange(period: string): { range: DateRange; label: string } {
+  const now = new Date()
+  const end = new Date(
+    now.getFullYear(),
+    now.getMonth(),
+    now.getDate(),
+    END_OF_DAY_HOURS,
+    END_OF_DAY_MINUTES,
+    END_OF_DAY_SECONDS,
+    END_OF_DAY_MS,
+  )
+
+  switch (period) {
+    case 'today': {
+      const start = new Date(now.getFullYear(), now.getMonth(), now.getDate())
+      return { range: { start, end }, label: `Today (${toDateString(start)})` }
+    }
+    case 'yesterday': {
+      const start = new Date(now.getFullYear(), now.getMonth(), now.getDate() - 1)
+      const yesterdayEnd = new Date(now.getFullYear(), now.getMonth(), now.getDate() - 1, END_OF_DAY_HOURS, END_OF_DAY_MINUTES, END_OF_DAY_SECONDS, END_OF_DAY_MS)
+      return { range: { start, end: yesterdayEnd }, label: `Yesterday (${toDateString(start)})` }
+    }
+    case 'week': {
+      const start = new Date(now.getFullYear(), now.getMonth(), now.getDate() - 7)
+      return { range: { start, end }, label: 'Last 7 Days' }
+    }
+    case 'month': {
+      const start = new Date(now.getFullYear(), now.getMonth(), 1)
+      return { range: { start, end }, label: `${now.toLocaleString('default', { month: 'long' })} ${now.getFullYear()}` }
+    }
+    case '30days': {
+      const start = new Date(now.getFullYear(), now.getMonth(), now.getDate() - 30)
+      return { range: { start, end }, label: 'Last 30 Days' }
+    }
+    case 'all': {
+      const start = new Date(now.getFullYear(), now.getMonth() - ALL_TIME_MONTHS, now.getDate())
+      return { range: { start, end }, label: 'Last 6 months' }
+    }
+    default: {
+      const start = new Date(now.getFullYear(), now.getMonth(), now.getDate() - 7)
+      return { range: { start, end }, label: 'Last 7 Days' }
+    }
+  }
+}