Add custom npm registry/proxy support for frontend builds

Added configurable npm registry support to enable use of custom npm proxies or private registries during Docker builds. This is essential for corporate environments, air-gapped deployments, or when using npm mirrors.

**Changes:**
- Dockerfile.frontend: Added NPM_REGISTRY build argument with conditional configuration
- docker-compose.yml: Pass NPM_REGISTRY from environment to build args
- .env.example: Added NPM_REGISTRY configuration with usage examples

**Usage:**
Set NPM_REGISTRY in .env file or as environment variable:
- Nexus: http://nexus.company.com:8081/repository/npm-proxy/
- Artifactory: https://artifactory.company.com/artifactory/api/npm/npm-remote/
- Verdaccio: http://localhost:4873/
- Default: Leave blank for https://registry.npmjs.org/

**Example:**
```bash
NPM_REGISTRY=http://your-npm-proxy:8081/repository/npm-proxy/ ./quickstart.sh
```

Defaults to official npm registry if not specified.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

docker-compose.yml

@@ -61,6 +61,8 @@ services:
     build:
       context: .
       dockerfile: Dockerfile.frontend
+      args:
+        NPM_REGISTRY: ${NPM_REGISTRY:-https://registry.npmjs.org/}
     ports:
       - "80:80"
     depends_on: