fix(toc): scope the hide-on-mobile rule to the desktop TOC only

The previous rule hid every `.table-of-contents` on narrow
viewports, which also matched the React component re-rendered
inside `.toc-mobile-sidebar` (the hamburger drawer). Result:
the drawer opened to an empty body.

Scope the hide to `.content-grid .table-of-contents` so the
drawer copy of the same component stays visible.

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/src/styles/_layout.css

@@ -34,7 +34,11 @@
     grid-template-columns: 1fr;
   }
 
-  .table-of-contents {
+  /* Hide only the desktop TOC (the one rendered inside .content-grid).
+     We must NOT match the editor's mobile drawer, which renders the same
+     React TableOfContents component inside .toc-mobile-sidebar - matching
+     it here would empty the hamburger menu on narrow viewports. */
+  .content-grid .table-of-contents {
     position: static;
     display: none;
   }