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/src/test/canary.test.ts

@@ -0,0 +1,7 @@
+import { describe, it, expect } from 'vitest';
+
+describe('vitest canary', () => {
+  it('arithmetic still works', () => {
+    expect(1 + 1).toBe(2);
+  });
+});