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/AddToFolderMenu.tsx

@@ -93,6 +93,7 @@ export function AddToFolderMenu({ treeId, onFolderCreated }: AddToFolderMenuProp
           'hover:bg-white/10 hover:text-white'
         )}
         title="Add to folder"
+        aria-label="Add to folder"
       >
         <FolderPlus className="h-4 w-4" />
       </button>