Add markercluster for POIs and sync base layer via Yjs

- leaflet.markercluster: Dynamic import with fallback to plain layer
  group. Clusters POI markers at low zoom, ungroups at zoom 15+.
- Base layer sync: baselayerchange event writes to Yjs, new
  participants load the selected base layer on connect.
- Both tile overlays and base layer now persist across participants
  and crash recovery.

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

apps/planner/package.json

@@ -38,9 +38,11 @@
   "devDependencies": {
     "@react-router/dev": "catalog:",
     "@tailwindcss/vite": "catalog:",
+    "@types/leaflet.markercluster": "^1.5.6",
     "@types/react": "catalog:",
     "@types/react-dom": "catalog:",
     "@types/ws": "^8.18.1",
+    "leaflet.markercluster": "^1.5.3",
     "pino-pretty": "^13.1.3",
     "tailwindcss": "catalog:",
     "typescript": "catalog:",