test: add vitest test infrastructure and configuration

- Add vitest.config.ts with coverage thresholds
- Add test/setup.ts with testing-library setup
- Add test/helpers.ts with test utilities

admin-web/vitest.config.ts

@@ -0,0 +1,37 @@
+import { defineConfig } from 'vitest/config'
+import react from '@vitejs/plugin-react'
+import path from 'path'
+
+export default defineConfig({
+  plugins: [react()],
+  test: {
+    environment: 'jsdom',
+    globals: true,
+    setupFiles: ['./test/setup.ts'],
+    include: ['**/*.{test,spec}.{js,mjs,cjs,ts,mts,cts,jsx,tsx}'],
+    coverage: {
+      provider: 'v8',
+      reporter: ['text', 'json', 'html'],
+      exclude: [
+        'node_modules/',
+        'test/',
+        '**/*.d.ts',
+        '**/*.config.*',
+        '**/mock/**',
+      ],
+      thresholds: {
+        lines: 88,
+        functions: 90,
+        branches: 78,
+        statements: 85,
+      },
+    },
+    testTimeout: 30000,
+    hookTimeout: 30000,
+  },
+  resolve: {
+    alias: {
+      '@': path.resolve(__dirname, './'),
+    },
+  },
+})