Story 28: Show remaining test TODOs in the UI

Add TodoPanel that displays unchecked acceptance criteria from current
story files. Backend parses `- [ ]` lines from markdown, frontend
shows them in a panel with refresh. Includes 4 Rust unit tests,
3 Vitest tests, 3 Playwright E2E tests.

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

frontend/src/App.test.tsx

@@ -43,6 +43,9 @@ vi.mock("./api/workflow", () => {
 			recordTests: vi.fn(),
 			ensureAcceptance: vi.fn(),
 			getReviewQueue: vi.fn(),
+			collectCoverage: vi.fn(),
+			recordCoverage: vi.fn(),
+			getStoryTodos: vi.fn().mockResolvedValue({ stories: [] }),
 		},
 	};
 });