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/store/userPreferencesStore.test.ts

@@ -0,0 +1,45 @@
+import { describe, it, expect, beforeEach } from 'vitest'
+import { useUserPreferencesStore } from './userPreferencesStore'
+
+describe('userPreferencesStore', () => {
+  beforeEach(() => {
+    // Reset store to defaults
+    useUserPreferencesStore.setState({
+      defaultExportFormat: 'markdown',
+      treeLibraryView: 'grid',
+      treeLibrarySortBy: 'usage_count',
+    })
+  })
+
+  it('has correct defaults', () => {
+    const state = useUserPreferencesStore.getState()
+    expect(state.defaultExportFormat).toBe('markdown')
+    expect(state.treeLibraryView).toBe('grid')
+    expect(state.treeLibrarySortBy).toBe('usage_count')
+  })
+
+  it('sets export format', () => {
+    useUserPreferencesStore.getState().setDefaultExportFormat('html')
+    expect(useUserPreferencesStore.getState().defaultExportFormat).toBe('html')
+  })
+
+  it('sets tree library view', () => {
+    useUserPreferencesStore.getState().setTreeLibraryView('list')
+    expect(useUserPreferencesStore.getState().treeLibraryView).toBe('list')
+  })
+
+  it('sets tree library view to table', () => {
+    useUserPreferencesStore.getState().setTreeLibraryView('table')
+    expect(useUserPreferencesStore.getState().treeLibraryView).toBe('table')
+  })
+
+  it('sets sort by', () => {
+    useUserPreferencesStore.getState().setTreeLibrarySortBy('name')
+    expect(useUserPreferencesStore.getState().treeLibrarySortBy).toBe('name')
+  })
+
+  it('sets sort by to created_at', () => {
+    useUserPreferencesStore.getState().setTreeLibrarySortBy('created_at')
+    expect(useUserPreferencesStore.getState().treeLibrarySortBy).toBe('created_at')
+  })
+})