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/pages/TreeEditorPage.tsx

@@ -10,7 +10,7 @@ import { TreeEditorLayout } from '@/components/tree-editor/TreeEditorLayout'
 import { ValidationSummary } from '@/components/tree-editor/ValidationSummary'
 import { useKeyboardShortcuts } from '@/hooks/useKeyboardShortcuts'
 import { usePermissions } from '@/hooks/usePermissions'
-import { cn } from '@/lib/utils'
+import { cn, safeGetItem } from '@/lib/utils'
 import { toast } from '@/lib/toast'
 
 export function TreeEditorPage() {
@@ -156,7 +156,7 @@ export function TreeEditorPage() {
         initNewTree()
         setTreeStatus('draft') // New trees start as draft
         // Check for draft after initializing
-        const draftExists = localStorage.getItem('tree-editor-draft') !== null
+        const draftExists = safeGetItem('tree-editor-draft') !== null
         if (draftExists) {
           setShowDraftPrompt(true)
         }