fix: split category fetch, safe localStorage, aria-labels on icon buttons

- TreeLibraryPage: split categories into a mount-only fetch so filter
  changes only re-fetch trees (not categories every time)
- Add safeGetItem/safeSetItem/safeRemoveItem helpers in utils.ts to
  prevent crashes in private browsing or when storage is unavailable
- Replace raw localStorage calls in ScratchpadSidebar, TreeNavigationPage,
  TreeEditorPage, and treeEditorStore with safe wrappers
- Add aria-label to 20 icon-only buttons across 8 component files
  for screen reader accessibility

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/components/library/TreeTableView.tsx

@@ -192,6 +192,7 @@ export function TreeTableView({
                         'hover:bg-white/10 hover:text-white'
                       )}
                       title="Fork tree"
+                      aria-label="Fork tree"
                     >
                       <GitBranch className="h-3.5 w-3.5" />
                     </button>
@@ -204,6 +205,7 @@ export function TreeTableView({
                         'hover:bg-white/10 hover:text-white'
                       )}
                       title="Edit tree"
+                      aria-label="Edit tree"
                     >
                       <Pencil className="h-3.5 w-3.5" />
                     </Link>