Ports the wireframe window shell into React with exact markup/classes:
- Shared components: Window, Sidebar (traffic lights, gradient mark, six
nav items with ⌘ keycaps + active `on` rail, status line), TopBar
(title, scope, period SegTabs Today/7D/30D/Month/6M/Custom, ProviderPop),
Panel, Stat, SegTabs, ProviderPop, Hint.
- usePolled(fetcher, deps): generic 30s poll + loading/error state,
errors normalized to CliError.
- App.tsx: local section state switches the content outlet; the Overview
placeholder round-trips getOverview('30days','all') and renders the raw
current.cost, with an honest first-run "locate CLI" state when the
codeburn binary isn't found (never a crash).
- Sidebar.test.tsx: six nav items, click routes onNavigate, active `on`.
