chore(ui): add Vitest + Testing Library scaffolding

Prepares for Plan 03 unit tests (MustacheEditor, NotificationBell, wizard step
validation). jsdom environment + jest-dom matchers + canary test verifies the
wiring.

ui/vitest.config.ts

@@ -0,0 +1,14 @@
+import { defineConfig } from 'vitest/config';
+import react from '@vitejs/plugin-react';
+
+export default defineConfig({
+  plugins: [react()],
+  test: {
+    environment: 'jsdom',
+    globals: true,
+    setupFiles: ['./src/test/setup.ts'],
+    include: ['src/**/*.test.{ts,tsx}'],
+    exclude: ['src/test/e2e/**', 'node_modules/**'],
+    css: true,
+  },
+});