refactor(comments): tidy comments panel header and quote layout

- Merge the comments side-panel header into the Open/Resolved tab row,
  then drop the now-redundant "Comments" title; the panel keeps its
  accessible name via the AppShell.Aside aria-label.
- Overlay the close (X) button on the right of the tab row and nudge it
  up 4px to align with the tab labels; the tab list stays full-width so
  its bottom border line is preserved. The toc/details tabs keep their
  existing shared header and scroll area unchanged.
- Quote block (.textSelection): increase top margin (2px -> 8px) so it
  no longer sticks to the timestamp when it is the first block, and add
  margin-left: 6px so the quote's left bar lines up with the comment
  body text left edge.

apps/client/src/features/comment/components/comment.module.css

@@ -3,7 +3,12 @@
 }
 
 .textSelection {
-    margin-top: 2px;
+    /* Breathing room below the comment header (author + timestamp) so the
+       quote does not stick to the timestamp when it is the first block. */
+    margin-top: 8px;
+    /* Align the quote's left bar with the comment body text left edge
+       (the comment editor insets its text by 6px). */
+    margin-left: 6px;
     border-left: 2px solid var(--mantine-color-gray-6);
     padding: 6px;
     background: var(--mantine-color-gray-light);