test: set up Vitest with 61 frontend tests

Add Vitest + testing-library/react + jsdom for frontend testing.
Tests cover: cn() utility (6), usePermissions hook (27), useTreeValidation
hook (22), and userPreferencesStore (6). CI updated to run frontend tests.

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

.github/workflows/ci.yml

@@ -89,5 +89,8 @@ jobs:
       - name: Lint
         run: cd frontend && npm run lint
 
+      - name: Test
+        run: cd frontend && npm test
+
       - name: Build
         run: cd frontend && npm run build