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/TreeGridView.tsx

@@ -85,6 +85,7 @@ export function TreeGridView({
                     'hover:bg-white/10 hover:text-white'
                   )}
                   title="Fork tree"
+                  aria-label="Fork tree"
                 >
                   <GitBranch className="h-4 w-4" />
                 </button>
@@ -97,6 +98,7 @@ export function TreeGridView({
                     'hover:bg-white/10 hover:text-white'
                   )}
                   title="Edit tree"
+                  aria-label="Edit tree"
                 >
                   <Pencil className="h-4 w-4" />
                 </Link>
@@ -110,6 +112,7 @@ export function TreeGridView({
                     'hover:bg-red-400/10 hover:text-red-400'
                   )}
                   title="Delete tree"
+                  aria-label="Delete tree"
                 >
                   <Trash2 className="h-4 w-4" />
                 </button>