ci: bake NEXT_PUBLIC_MAP_TILE_URL into the frontend build (#117)

The map override var was added to MapView.tsx but never passed as a build-arg, so prod still shipped the dead Carto tile URL. Point it at OSM raster tiles.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Claude-Session: https://claude.ai/code/session_01Tbknwttzou4s46PAykmtix

.gitea/workflows/deploy.yml

@@ -63,6 +63,7 @@ jobs:
             NEXT_PUBLIC_FIREBASE_MESSAGING_SENDER_ID=${{ secrets.FIREBASE_MESSAGING_SENDER_ID }}
             NEXT_PUBLIC_FIREBASE_APP_ID=${{ secrets.FIREBASE_APP_ID }}
             NEXT_PUBLIC_FIRESTORE_DATABASE=${{ secrets.FIRESTORE_DATABASE }}
+            NEXT_PUBLIC_MAP_TILE_URL=https://tile.openstreetmap.org/{z}/{x}/{y}.png
 
   deploy:
     name: Deploy to VM