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>

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>

frontend/src/components/library/TreeListView.tsx

@@ -88,6 +88,7 @@ export function TreeListView({
                     'hover:bg-white/10 hover:text-white'
                   )}
                   title="Fork tree"
+                  aria-label="Fork tree"
                 >
                   <GitBranch className="h-4 w-4" />
                 </button>
@@ -100,6 +101,7 @@ export function TreeListView({
                     'hover:bg-white/10 hover:text-white'
                   )}
                   title="Edit tree"
+                  aria-label="Edit tree"
                 >
                   <Pencil className="h-4 w-4" />
                 </Link>

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>

frontend/src/components/library/ViewToggle.tsx

@@ -22,6 +22,7 @@ export function ViewToggle({ view, onChange, className }: ViewToggleProps) {
             : 'text-white/50 hover:bg-white/[0.06] hover:text-white'
         )}
         title="Grid view"
+        aria-label="Grid view"
       >
         <LayoutGrid className="h-4 w-4" />
       </button>
@@ -35,6 +36,7 @@ export function ViewToggle({ view, onChange, className }: ViewToggleProps) {
             : 'text-white/50 hover:bg-white/[0.06] hover:text-white'
         )}
         title="List view"
+        aria-label="List view"
       >
         <List className="h-4 w-4" />
       </button>
@@ -48,6 +50,7 @@ export function ViewToggle({ view, onChange, className }: ViewToggleProps) {
             : 'text-white/50 hover:bg-white/[0.06] hover:text-white'
         )}
         title="Table view"
+        aria-label="Table view"
       >
         <Table className="h-4 w-4" />
       </button>