fix(sidebar): make entire section header row clickable

The toggle was only on the chevron button. Now the full row
(chevron + icon + label) triggers onToggle on click or Enter/Space.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/design-system/layout/Sidebar/Sidebar.module.css

@@ -215,6 +215,8 @@
   gap: 2px;
   width: 100%;
   padding: 8px 0 4px;
+  cursor: pointer;
+  user-select: none;
 }
 
 .treeSectionChevronBtn {