Add frontend Dockerfile and integrate into docker-compose

- Multi-stage Dockerfile (deps → build → runner) for optimized production image
- Enable Next.js standalone output for minimal container size
- Add .dockerignore to exclude dev artifacts from build context
- Add frontend service to docker-compose.yml with API dependency

https://claude.ai/code/session_01QU6gpDgMtX4b9k1UXTivEf

docker-compose.yml

@@ -49,6 +49,21 @@ services:
         condition: service_healthy
     command: uvicorn main:app --host 0.0.0.0 --port 8000 --reload
 
+  # ---------------------------------------------------------------------------
+  # CouncilOS Frontend — Next.js + React Flow UI
+  # ---------------------------------------------------------------------------
+  frontend:
+    build:
+      context: ./frontend
+      dockerfile: Dockerfile
+    restart: unless-stopped
+    ports:
+      - "3000:3000"
+    environment:
+      NEXT_PUBLIC_API_URL: http://api:8000
+    depends_on:
+      - api
+
 volumes:
   postgres_data:
   chroma_data: