infra: Add multi-stage frontend build to Docker

Nginx container now builds the Vue frontend in a Node stage
instead of mounting local dist/ files. This means:
- No need to commit dist/ or build locally before deploying
- docker compose up --build handles everything end-to-end
- Removed obsolete compose version key

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docker/docker-compose.yml

@@ -1,5 +1,3 @@
-version: '3.8'
-
 services:
   postgres:
     image: postgres:16-alpine
@@ -61,13 +59,14 @@ services:
       - "8088:3000"
 
   nginx:
-    image: nginx:alpine
+    build:
+      context: ../frontend
+      dockerfile: ../docker/Dockerfile.nginx
     container_name: corrosion-nginx
     ports:
       - "8087:80"
     volumes:
       - ./nginx.conf:/etc/nginx/nginx.conf:ro
-      - ../frontend/dist:/usr/share/nginx/html:ro
       - map_data:/data/maps:ro
     depends_on:
       - api