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/tree-editor/DynamicArrayField.tsx

@@ -53,6 +53,7 @@ export function DynamicArrayField<T>({
                 disabled={index === 0}
                 className="rounded p-0.5 text-white/50 hover:bg-white/[0.06] hover:text-white disabled:opacity-30"
                 title="Move up"
+                aria-label="Move up"
               >
                 <ChevronUp className="h-3 w-3" />
               </button>
@@ -62,6 +63,7 @@ export function DynamicArrayField<T>({
                 disabled={index === items.length - 1}
                 className="rounded p-0.5 text-white/50 hover:bg-white/[0.06] hover:text-white disabled:opacity-30"
                 title="Move down"
+                aria-label="Move down"
               >
                 <ChevronDown className="h-3 w-3" />
               </button>
@@ -78,6 +80,7 @@ export function DynamicArrayField<T>({
               onClick={() => onRemove(index)}
               className="mt-1 rounded p-1 text-white/50 hover:bg-red-400/20 hover:text-red-400"
               title="Remove"
+              aria-label="Remove"
             >
               <Trash2 className="h-4 w-4" />
             </button>