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/TreeNavigationPage.tsx

@@ -4,7 +4,7 @@ import { treesApi, sessionsApi } from '@/api'
 import { useTreeNavigationShortcuts } from '@/hooks/useKeyboardShortcuts'
 import { useCustomStepFlow } from '@/hooks/useCustomStepFlow'
 import type { Tree, Session, DecisionRecord, TreeStructure } from '@/types'
-import { cn } from '@/lib/utils'
+import { cn, safeGetItem } from '@/lib/utils'
 import { MarkdownContent } from '@/components/ui/MarkdownContent'
 import { CustomStepModal } from '@/components/step-library/CustomStepModal'
 import { PostStepActionModal, ContinuationModal, ForkTreeModal, ScratchpadSidebar } from '@/components/session'
@@ -37,7 +37,7 @@ export function TreeNavigationPage() {
 
   // Scratchpad state
   const [scratchpadOpen, setScratchpadOpen] = useState(() => {
-    return localStorage.getItem('scratchpad-collapsed') === 'false'
+    return safeGetItem('scratchpad-collapsed') === 'false'
   })
 
   const findNode = (nodeId: string, structure?: TreeStructure): TreeStructure | null => {