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>

pnpm-lock.yaml

@@ -361,6 +361,9 @@ importers:
       '@tailwindcss/vite':
         specifier: 'catalog:'
         version: 4.2.2(vite@8.0.8(@types/node@25.5.2)(esbuild@0.27.4)(jiti@2.6.1)(tsx@4.21.0))
+      '@types/leaflet.markercluster':
+        specifier: ^1.5.6
+        version: 1.5.6
       '@types/react':
         specifier: 'catalog:'
         version: 19.2.14
@@ -370,6 +373,9 @@ importers:
       '@types/ws':
         specifier: ^8.18.1
         version: 8.18.1
+      leaflet.markercluster:
+        specifier: ^1.5.3
+        version: 1.5.3(leaflet@1.9.4)
       pino-pretty:
         specifier: ^13.1.3
         version: 13.1.3
@@ -2094,6 +2100,9 @@ packages:
   '@types/json-schema@7.0.15':
     resolution: {integrity: sha512-5+fP8P8MFNC+AyZCDxrB2pkZFPGzqQWUzpSeuuVLvm8VMcorNYavBqoFcxK8bQz4Qsbn4oUEEem4wDLfcysGHA==}
 
+  '@types/leaflet.markercluster@1.5.6':
+    resolution: {integrity: sha512-I7hZjO2+isVXGYWzKxBp8PsCzAYCJBc29qBdFpquOCkS7zFDqUsUvkEOyQHedsk/Cy5tocQzf+Ndorm5W9YKTQ==}
+
   '@types/leaflet@1.9.21':
     resolution: {integrity: sha512-TbAd9DaPGSnzp6QvtYngntMZgcRk+igFELwR2N99XZn7RXUdKgsXMR+28bUO0rPsWp8MIu/f47luLIQuSLYv/w==}
 
@@ -2992,6 +3001,11 @@ packages:
   keyv@4.5.4:
     resolution: {integrity: sha512-oxVHkHR/EJf2CNXnWxRLW6mg7JyCCUcG0DtEGmL2ctUo1PNTin1PUil+r/+4r5MpVgC/fn1kjsx7mjSujKqIpw==}
 
+  leaflet.markercluster@1.5.3:
+    resolution: {integrity: sha512-vPTw/Bndq7eQHjLBVlWpnGeLa3t+3zGiuM7fJwCkiMFq+nmRuG3RI3f7f4N4TDX7T4NpbAXpR2+NTRSEGfCSeA==}
+    peerDependencies:
+      leaflet: ^1.3.1
+
   leaflet@1.9.4:
     resolution: {integrity: sha512-nxS1ynzJOmOlHp+iL3FyWqK89GtNL8U8rvlMOsQdTTssxZwCXh8N2NB3GDQOL+YR3XnWyZAxwQixURb+FA74PA==}
 
@@ -5598,6 +5612,10 @@ snapshots:
 
   '@types/json-schema@7.0.15': {}
 
+  '@types/leaflet.markercluster@1.5.6':
+    dependencies:
+      '@types/leaflet': 1.9.21
+
   '@types/leaflet@1.9.21':
     dependencies:
       '@types/geojson': 7946.0.16
@@ -6542,6 +6560,10 @@ snapshots:
     dependencies:
       json-buffer: 3.0.1
 
+  leaflet.markercluster@1.5.3(leaflet@1.9.4):
+    dependencies:
+      leaflet: 1.9.4
+
   leaflet@1.9.4: {}
 
   levn@0.4.1: