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

.dockerignore

@@ -0,0 +1,11 @@
+node_modules
+.git
+.gitignore
+.vscode
+.idea
+*.md
+!README.md
+.env
+.env.*
+dist
+.astro