test: fix react 19 compatibility and create vitest configuration

- Upgrade @testing-library/react from ^14.0.0 to ^16.0.0 (supports React 19)
- Create vitest.config.ts with jsdom environment, v8 coverage, and 80% coverage thresholds
- Run clean npm install - 829 packages added successfully
- Vitest 1.6.1 verified and operational

frontend/vitest.config.ts

@@ -0,0 +1,35 @@
+import { defineConfig } from 'vitest/config'
+import react from '@vitejs/plugin-react'
+import path from 'path'
+
+export default defineConfig({
+  plugins: [react()],
+  resolve: {
+    alias: {
+      '@': path.resolve(__dirname, './'),
+    },
+  },
+  test: {
+    globals: true,
+    environment: 'jsdom',
+    setupFiles: ['./tests/setup.ts'],
+    coverage: {
+      provider: 'v8',
+      reporter: ['text', 'html'],
+      all: true,
+      lines: 80,
+      functions: 80,
+      branches: 80,
+      statements: 80,
+      include: ['components/**', 'hooks/**', 'lib/**', 'app/**'],
+      exclude: [
+        'node_modules/',
+        'tests/',
+        '.next/',
+        'coverage/',
+        '**/*.test.tsx',
+        '**/*.spec.ts',
+      ],
+    },
+  },
+})