feat: add frontend dev container to docker-compose

- frontend/Dockerfile: Node 20 Alpine image running npm run dev
- docker-compose: frontend service with depends_on db/minio/backend,
  source mounted as volume, named volume for node_modules to avoid
  OS binary conflicts between host and container
- vite.config.ts: make API proxy target configurable via
  API_PROXY_TARGET env var (defaults to localhost:8080 for local dev,
  set to http://backend:8080 inside Docker)
- .env: update PORT_FRONTEND to 5173 (actual vite dev server port)

Usage:
  docker compose up frontend   # starts frontend + all dependencies
  docker compose up            # starts everything

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

frontend/vite.config.ts

@@ -12,7 +12,7 @@ export default defineConfig({
 		// Proxy für API-Aufrufe während der Entwicklung (Browser -> Vite -> Spring Boot)
 		proxy: {
 			'/api': {
-				target: 'http://localhost:8080',
+				target: process.env.API_PROXY_TARGET || 'http://localhost:8080',
 				changeOrigin: true
 			}
 		}