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.tsx

@@ -62,6 +62,11 @@ function renderMarks(
         return <em key={key}>{acc}</em>;
       case "strike":
         return <s key={key}>{acc}</s>;
+      case "underline":
+        // StarterKit enables the Underline extension by default (Mod-u) and
+        // CommentEditor does not disable it, so real comments can carry this
+        // mark. Render it here rather than degrading the whole comment.
+        return <u key={key}>{acc}</u>;
       case "code":
         return <code key={key}>{acc}</code>;
       case "link": {