Scaffold project foundation

Fresh start: Vite + React 19 + strict TS, Tailwind v4, PWA, Vitest/Playwright.
Replaces the prior DeepSeek-built app (deleted; ~135 documented bugs, no VCS).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

vitest.setup.ts

@@ -0,0 +1,8 @@
+import '@testing-library/jest-dom/vitest';
+import 'fake-indexeddb/auto';
+import { cleanup } from '@testing-library/react';
+import { afterEach } from 'vitest';
+
+afterEach(() => {
+  cleanup();
+});