[QOTD-5] Configure test runner with Vitest and placeholder test

Add src/placeholder.test.ts to confirm the Vitest harness works
end-to-end (npm test passes). Update CLAUDE.md project structure
to reflect the now-created src/ directory.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

CLAUDE.md

@@ -39,7 +39,8 @@ npm run dev
 
 ```
 quote-of-the-day-api/
-├── src/               # TypeScript source (to be created)
+├── src/               # TypeScript source
+│   └── placeholder.test.ts  # harness smoke test
 ├── dist/              # compiled output (git-ignored)
 ├── package.json
 ├── package-lock.json

src/placeholder.test.ts

@@ -0,0 +1,7 @@
+import { describe, it, expect } from 'vitest';
+
+describe('test harness', () => {
+  it('runs a passing test', () => {
+    expect(1 + 1).toBe(2);
+  });
+});