feat(#375): frontend token usage and cost dashboard

- Install recharts for data visualization
- Add Usage nav item to sidebar navigation
- Create telemetry API service with data fetching functions
- Build dashboard page with summary cards, charts, and time range selector
- Token usage line chart, cost breakdown bar chart, task outcome pie chart
- Loading and empty states handled
- Responsive layout with PDA-friendly design
- Add unit tests (14 tests passing)

Refs #375

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/web/src/components/layout/Navigation.tsx

@@ -16,6 +16,7 @@ export function Navigation(): React.JSX.Element {
     { href: "/tasks", label: "Tasks" },
     { href: "/calendar", label: "Calendar" },
     { href: "/knowledge", label: "Knowledge" },
+    { href: "/usage", label: "Usage" },
   ];
 
   // Global keyboard shortcut for search (Cmd+K or Ctrl+K)