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/tsconfig.app.json

@@ -29,5 +29,6 @@
       "@/*": ["./src/*"]
     }
   },
-  "include": ["src"]
+  "include": ["src"],
+  "exclude": ["src/**/*.test.ts", "src/**/*.test.tsx", "src/**/*.spec.ts", "src/**/*.spec.tsx", "src/test"]
 }