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/vite.config.ts

@@ -1,3 +1,4 @@
+/// <reference types="vitest/config" />
 import { defineConfig } from 'vite'
 import react from '@vitejs/plugin-react'
 import path from 'path'
@@ -10,6 +11,12 @@ export default defineConfig({
       '@': path.resolve(__dirname, './src'),
     },
   },
+  test: {
+    globals: true,
+    environment: 'jsdom',
+    setupFiles: './src/test/setup.ts',
+    include: ['src/**/*.{test,spec}.{ts,tsx}'],
+  },
   build: {
     rollupOptions: {
       output: {