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>

server/src/io/story_metadata.rs

@@ -59,6 +59,18 @@ fn build_metadata(front: FrontMatter) -> StoryMetadata {
     }
 }
 
+pub fn parse_unchecked_todos(contents: &str) -> Vec<String> {
+    contents
+        .lines()
+        .filter_map(|line| {
+            let trimmed = line.trim();
+            trimmed
+                .strip_prefix("- [ ] ")
+                .map(|text| text.to_string())
+        })
+        .collect()
+}
+
 fn parse_test_plan_status(value: &str) -> TestPlanStatus {
     match value {
         "approved" => TestPlanStatus::Approved,
@@ -108,4 +120,31 @@ workflow: tdd
             Err(StoryMetaError::InvalidFrontMatter(_))
         ));
     }
+
+    #[test]
+    fn parse_unchecked_todos_mixed() {
+        let input = "## AC\n- [ ] First thing\n- [x] Done thing\n- [ ] Second thing\n";
+        assert_eq!(
+            parse_unchecked_todos(input),
+            vec!["First thing", "Second thing"]
+        );
+    }
+
+    #[test]
+    fn parse_unchecked_todos_all_checked() {
+        let input = "- [x] Done\n- [x] Also done\n";
+        assert!(parse_unchecked_todos(input).is_empty());
+    }
+
+    #[test]
+    fn parse_unchecked_todos_no_checkboxes() {
+        let input = "# Story\nSome text\n- A bullet\n";
+        assert!(parse_unchecked_todos(input).is_empty());
+    }
+
+    #[test]
+    fn parse_unchecked_todos_leading_whitespace() {
+        let input = "  - [ ] Indented item\n";
+        assert_eq!(parse_unchecked_todos(input), vec!["Indented item"]);
+    }
 }