fix(comment): underline mark + draft-surviving tabs + test coverage (#349 review F1-F4)

- F1: render the `underline` mark statically (StarterKit v3 enables Underline;
  comment-editor does not disable it) — an underlined comment no longer degrades
  the whole comment to the read-only editor fallback. renderMarks gains a
  `case "underline" -> <u>`, mirroring the other marks (+ test).
- F2: keep the Open tab panel mounted (`Tabs.Panel value="open" keepMounted`)
  while the heavy Resolved panel still unmounts (`Tabs keepMounted={false}`). A
  per-panel keepMounted overrides the parent's `false` (Mantine 8 TabsPanel), so
  an in-progress reply draft / edit in the Open panel survives an
  Open->Resolved->Open switch, keeping the micro-opt of not mounting the large
  Resolved list.
- F3: cover edit->save->re-render in comment-list-item.test.tsx — save calls
  mutateAsync with JSON.stringify(editContentRef) and a new comment.content prop
  updates the visible body; cancel restores the static body without mutating;
  clearing editContentRef after cancel.
- F4: extract childrenByParent grouping into an exported pure
  `buildChildrenByParent(items)` (unit-tested: nesting, orphan reply, sibling
  order) + new comment-list-with-tabs.test.tsx covering the lazy reply-editor
  activation (stub -> click/focus/Enter mounts the editor).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/client/src/features/comment/components/comment-content-view.test.tsx

@@ -72,6 +72,15 @@ describe("CommentContentView", () => {
     expect(container.querySelector("s")?.textContent).toBe("st");
   });
 
+  it("renders the underline mark as <u> (not the editor fallback)", () => {
+    const { container } = renderView(
+      doc([para([text("un", [{ type: "underline" }])])]),
+    );
+    expect(container.querySelector("u")?.textContent).toBe("un");
+    // Underline is a supported mark, so no degrade to the editor fallback.
+    expect(screen.queryByTestId("comment-editor-fallback")).toBeNull();
+  });
+
   it("renders the code mark as <code>", () => {
     const { container } = renderView(
       doc([para([text("co", [{ type: "code" }])])]),