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/TreeLibraryPage.tsx

@@ -58,8 +58,16 @@ export function TreeLibraryPage() {
     }
   }, [])
 
+  // Load categories once on mount (they rarely change)
   useEffect(() => {
-    loadData()
+    categoriesApi.list()
+      .then(setCategories)
+      .catch((err) => console.error('Failed to load categories:', err))
+  }, [])
+
+  // Load trees when filters change
+  useEffect(() => {
+    loadTrees()
   }, [selectedCategoryId, selectedTags, selectedFolderId, treeLibrarySortBy, showDrafts])
 
   // Load folders on mount and listen for changes
@@ -70,21 +78,17 @@ export function TreeLibraryPage() {
     return () => window.removeEventListener('folder-changed', handleFolderChange)
   }, [loadFolders])
 
-  const loadData = async () => {
+  const loadTrees = async () => {
     setIsLoading(true)
     try {
-      const [treesData, categoriesData] = await Promise.all([
-        treesApi.list({
-          category_id: selectedCategoryId || undefined,
-          tags: selectedTags.length > 0 ? selectedTags.join(',') : undefined,
-          folder_id: selectedFolderId || undefined,
-          sort_by: treeLibrarySortBy,
-          include_drafts: showDrafts || undefined,
-        }),
-        categoriesApi.list(),
-      ])
+      const treesData = await treesApi.list({
+        category_id: selectedCategoryId || undefined,
+        tags: selectedTags.length > 0 ? selectedTags.join(',') : undefined,
+        folder_id: selectedFolderId || undefined,
+        sort_by: treeLibrarySortBy,
+        include_drafts: showDrafts || undefined,
+      })
       setTrees(treesData)
-      setCategories(categoriesData)
     } catch (err) {
       toast.error('Failed to load trees')
       console.error(err)
@@ -95,7 +99,7 @@ export function TreeLibraryPage() {
 
   const handleSearch = async () => {
     if (!searchQuery.trim()) {
-      loadData()
+      loadTrees()
       return
     }
     setIsLoading(true)
@@ -413,7 +417,7 @@ export function TreeLibraryPage() {
           setFolderModalOpen(false)
           setNewFolderParentId(null)
         }}
-        onSave={loadData}
+        onSave={loadTrees}
       />
 
       {/* Delete Confirmation */}