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 <[email protected]>

apps/web/package.json

@@ -33,6 +33,7 @@
     "react": "^19.0.0",
     "react-dom": "^19.0.0",
     "react-grid-layout": "^2.2.2",
+    "recharts": "^3.7.0",
     "socket.io-client": "^4.8.3"
   },
   "devDependencies": {