Add Gitea Actions deploy workflow and server configuration

- Add basePath /admin to Next.js config for path-based routing
- Add .gitea/workflows/deploy.yml for CI/CD via Gitea Actions

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

admin-ui/package.json

@@ -25,6 +25,7 @@
     "react-hook-form": "^7.71.1",
     "react-markdown": "^10.1.0",
     "react-syntax-highlighter": "^16.1.1",
+    "rehype-raw": "^7.0.0",
     "sonner": "^2.0.7",
     "tailwind-merge": "^3.5.0",
     "tailwindcss-animate": "^1.0.7"