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/tests/e2e/tdd-gates.spec.ts

@@ -81,6 +81,9 @@ function mockChatApis(
 		page.route("**/api/io/fs/list/absolute**", (route) =>
 			route.fulfill({ json: [] }),
 		),
+		page.route("**/api/workflow/todos", (route) =>
+			route.fulfill({ json: { stories: [] } }),
+		),
 	]);
 }