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/next.config.ts

@@ -4,4 +4,5 @@ const withNextIntl = createNextIntlPlugin('./i18n/request.ts');
 
 export default withNextIntl({
   output: 'standalone',
+  basePath: '/admin',
 });