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/admin/CategoryRow.tsx

@@ -81,6 +81,7 @@ export function CategoryRow({
             'hover:bg-white/10 hover:text-white'
           )}
           title="Edit category"
+          aria-label="Edit category"
         >
           <Edit className="h-4 w-4" />
         </button>
@@ -94,6 +95,7 @@ export function CategoryRow({
               'hover:bg-accent hover:text-accent-foreground'
             )}
             title="Archive category"
+            aria-label="Archive category"
           >
             <Archive className="h-4 w-4" />
           </button>
@@ -106,6 +108,7 @@ export function CategoryRow({
               'hover:bg-accent hover:text-accent-foreground'
             )}
             title="Restore category"
+            aria-label="Restore category"
           >
             <RotateCcw className="h-4 w-4" />
           </button>