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/components/selection/usePathCompletion.test.ts

@@ -1,5 +1,5 @@
 import { act, renderHook, waitFor } from "@testing-library/react";
-import { describe, expect, it, vi } from "vitest";
+import { beforeEach, describe, expect, it, vi } from "vitest";
 import type { FileEntry } from "./usePathCompletion";
 import {
 	getCurrentPartial,