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/lib/utils.test.ts

@@ -0,0 +1,28 @@
+import { describe, it, expect } from 'vitest'
+import { cn } from './utils'
+
+describe('cn', () => {
+  it('merges class names', () => {
+    expect(cn('foo', 'bar')).toBe('foo bar')
+  })
+
+  it('handles conditional classes', () => {
+    expect(cn('base', false && 'hidden', 'visible')).toBe('base visible')
+  })
+
+  it('deduplicates tailwind classes', () => {
+    expect(cn('p-4', 'p-2')).toBe('p-2')
+  })
+
+  it('handles undefined and null', () => {
+    expect(cn('foo', undefined, null, 'bar')).toBe('foo bar')
+  })
+
+  it('handles empty inputs', () => {
+    expect(cn()).toBe('')
+  })
+
+  it('merges conflicting tailwind utilities', () => {
+    expect(cn('text-red-500', 'text-blue-500')).toBe('text-blue-500')
+  })
+})