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/src/test/setup.ts

@@ -0,0 +1,16 @@
+import '@testing-library/jest-dom/vitest'
+
+// Mock localStorage for zustand persist middleware
+const localStorageMock = (() => {
+  let store: Record<string, string> = {}
+  return {
+    getItem: (key: string) => store[key] ?? null,
+    setItem: (key: string, value: string) => { store[key] = value },
+    removeItem: (key: string) => { delete store[key] },
+    clear: () => { store = {} },
+    get length() { return Object.keys(store).length },
+    key: (index: number) => Object.keys(store)[index] ?? null,
+  }
+})()
+
+Object.defineProperty(globalThis, 'localStorage', { value: localStorageMock })