Implement Planner map UI with collaborative editing (Groups 5+6)

Core Planner experience:
- Full-screen Leaflet map with OSM/OpenTopoMap/CyclOSM layers
- Click to add waypoints, drag to move, right-click to delete
- Waypoint sidebar with reorder, delete, route stats
- Real-time Yjs sync between participants
- Live cursors showing other users' mouse positions on the map

Routing:
- Host election via Yjs awareness (lowest client ID)
- Auto-failover when host disconnects
- Debounced BRouter route computation (500ms)
- Route polyline display from BRouter GeoJSON
- Route stats (distance, elevation gain/loss)

Client-side architecture:
- useYjs hook: WebSocket connection, awareness, document management
- useRouting hook: host election, route computation, stats parsing
- PlannerMap component: Leaflet + Yjs integration
- WaypointSidebar component: list with reorder/delete
- Lazy-loaded components for code splitting

Remaining: elevation chart, profile selector, GPX export (tasks 6.7-6.9)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

package.json

@@ -31,6 +31,7 @@
     "@tailwindcss/vite": "^4.2.2",
     "@testing-library/jest-dom": "^6.9.1",
     "@testing-library/react": "^16.3.2",
+    "@types/geojson": "^7946.0.16",
     "@types/leaflet": "^1.9.21",
     "@types/react": "^19.2.14",
     "@types/react-dom": "^19.2.3",