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/lib/utils.ts

@@ -4,3 +4,28 @@ import { twMerge } from 'tailwind-merge'
 export function cn(...inputs: ClassValue[]) {
   return twMerge(clsx(inputs))
 }
+
+/** Safe localStorage access — returns null on error (e.g. private browsing) */
+export function safeGetItem(key: string): string | null {
+  try {
+    return localStorage.getItem(key)
+  } catch {
+    return null
+  }
+}
+
+export function safeSetItem(key: string, value: string): void {
+  try {
+    localStorage.setItem(key, value)
+  } catch {
+    // Storage full or unavailable — silently fail
+  }
+}
+
+export function safeRemoveItem(key: string): void {
+  try {
+    localStorage.removeItem(key)
+  } catch {
+    // Storage unavailable — silently fail
+  }
+}