ci: add Gitea Actions CI/CD with Portainer deployment

Add complete CI/CD pipeline:
- Gitea Actions workflow (build + deploy)
- Multi-stage Dockerfile (Node build + nginx serve)
- nginx config with SPA routing and cache headers
- docker-compose.yml for local testing
- .dockerignore to optimize build

Pipeline flow:
1. Build job: npm ci + npm build + upload artifact
2. Deploy job (main only): Docker build + push to registry + Portainer webhook

Requires Gitea secrets:
- DOCKER_USERNAME
- DOCKER_PASSWORD
- PORTAINER_WEBHOOK_URL

nginx.conf

@@ -0,0 +1,51 @@
+user nginx;
+worker_processes auto;
+error_log /var/log/nginx/error.log warn;
+pid /var/run/nginx.pid;
+
+events {
+    worker_connections 1024;
+}
+
+http {
+    include /etc/nginx/mime.types;
+    default_type application/octet-stream;
+
+    log_format main '$remote_addr - $remote_user [$time_local] "$request" '
+                    '$status $body_bytes_sent "$http_referer" '
+                    '"$http_user_agent" "$http_x_forwarded_for"';
+
+    access_log /var/log/nginx/access.log main;
+
+    sendfile on;
+    tcp_nopush on;
+    keepalive_timeout 65;
+    gzip on;
+    gzip_vary on;
+    gzip_proxied any;
+    gzip_comp_level 6;
+    gzip_types text/plain text/css text/xml text/javascript application/json application/javascript application/xml+rss application/rss+xml font/truetype font/opentype application/vnd.ms-fontobject image/svg+xml;
+
+    server {
+        listen 80;
+        server_name _;
+        root /usr/share/nginx/html;
+        index index.html;
+
+        # Serve static files with cache headers
+        location ~* \.(js|css|png|jpg|jpeg|gif|ico|svg|woff|woff2|ttf|eot)$ {
+            expires 1y;
+            add_header Cache-Control "public, immutable";
+        }
+
+        # SPA fallback - serve index.html for all routes
+        location / {
+            try_files $uri $uri/ /index.html;
+        }
+
+        # Security headers
+        add_header X-Frame-Options "SAMEORIGIN" always;
+        add_header X-Content-Type-Options "nosniff" always;
+        add_header X-XSS-Protection "1; mode=block" always;
+    }
+}