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>

frontend/package.json

@@ -8,6 +8,8 @@
     "build": "tsc -b && vite build",
     "lint": "eslint .",
     "preview": "vite preview",
+    "test": "vitest run",
+    "test:watch": "vitest",
     "analyze": "vite-bundle-visualizer"
   },
   "dependencies": {
@@ -35,6 +37,9 @@
   },
   "devDependencies": {
     "@eslint/js": "^9.39.1",
+    "@testing-library/jest-dom": "^6.9.1",
+    "@testing-library/react": "^16.3.2",
+    "@testing-library/user-event": "^14.6.1",
     "@types/node": "^24.10.9",
     "@types/react": "^19.2.5",
     "@types/react-dom": "^19.2.3",
@@ -44,11 +49,13 @@
     "eslint-plugin-react-hooks": "^7.0.1",
     "eslint-plugin-react-refresh": "^0.4.24",
     "globals": "^16.5.0",
+    "jsdom": "^28.0.0",
     "postcss": "^8.5.6",
     "tailwindcss": "^3.4.19",
     "typescript": "~5.9.3",
     "typescript-eslint": "^8.46.4",
     "vite": "^7.2.4",
-    "vite-bundle-visualizer": "^1.2.1"
+    "vite-bundle-visualizer": "^1.2.1",
+    "vitest": "^4.0.18"
   }
 }