Add JS tests, refactor SitemapController, extract JS modules

- Extract mobile-menu.js and tabs.js from app.js
- Add Vitest tests with happy-dom and v8 coverage (100% on modules)
- Add JS test step to CI frontend and SonarQube workflows
- SonarQube: add JS lcov coverage report path
- SitemapController: extract URLSET_TEMPLATE constant, deduplicate methods

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitea/workflows/ci.yml

@@ -98,6 +98,9 @@ jobs:
       - name: Stylelint
         run: bunx stylelint "assets/**/*.scss"
 
+      - name: JS tests with coverage
+        run: bun run test:coverage
+
       - name: Build assets
         run: bun run build