feat(analytics): add usage analytics page with caching layer

- Add Analytics page with usage trends, model breakdown, sessions table
- Add server-side caching layer for better-ccusage data (TTL-based)
- Add request coalescing to prevent duplicate concurrent API calls
- Add /api/usage/refresh endpoint to manually clear cache
- Add date-range filter, summary cards, trend charts components
- Fix API parameter mismatch (since/until in YYYYMMDD format)
- Wire up Refresh button with loading state animation

ui/package.json

@@ -31,11 +31,14 @@
     "chokidar": "^5.0.0",
     "class-variance-authority": "^0.7.1",
     "clsx": "^2.1.1",
+    "date-fns": "^4.1.0",
     "lucide-react": "^0.556.0",
     "react": "^19.2.0",
+    "react-day-picker": "^9.12.0",
     "react-dom": "^19.2.0",
     "react-hook-form": "^7.68.0",
     "react-router-dom": "^7.10.1",
+    "recharts": "^2.12.0",
     "sonner": "^2.0.7",
     "tailwind-merge": "^3.4.0",
     "zod": "^4.1.13"
@@ -46,6 +49,7 @@
     "@types/node": "^24.10.1",
     "@types/react": "^19.2.5",
     "@types/react-dom": "^19.2.3",
+    "@types/recharts": "^1.8.29",
     "@vitejs/plugin-react": "^5.1.1",
     "eslint": "^9.39.1",
     "eslint-config-prettier": "^10.1.8",