feat: hierarchical notebooks (tree), remove all list view code, delete 22 unused files

- Add parentId to Notebook model (tree structure)
- Update sidebar to render parent/child notebooks with expand/collapse
- Add sub-notebook creation from parent notebook
- Remove 'list' from NotesViewMode type everywhere
- Delete 22 unused components, hooks, and UI files
- Wrap revalidatePath in try-catch to prevent save 500
- Update notebook API to support parentId in creation

memento-note/components/home-client.tsx

@@ -45,7 +45,7 @@ const NotebookSummaryDialog = dynamic(
 
 type InitialSettings = {
   showRecentNotes: boolean
-  notesViewMode: 'masonry' | 'tabs' | 'list'
+  notesViewMode: 'masonry' | 'tabs'
   noteHistory: boolean
   noteHistoryMode: 'manual' | 'auto'
   aiAssistantEnabled: boolean
@@ -65,7 +65,7 @@ export function HomeClient({ initialNotes, initialSettings }: HomeClientProps) {
   const [pinnedNotes, setPinnedNotes] = useState<Note[]>(
     initialNotes.filter(n => n.isPinned)
   )
-  const [notesViewMode, setNotesViewMode] = useState<NotesViewMode>(initialSettings.notesViewMode === 'list' ? 'masonry' : initialSettings.notesViewMode)
+  const [notesViewMode, setNotesViewMode] = useState<NotesViewMode>(initialSettings.notesViewMode)
   const [noteHistoryMode] = useState<'manual' | 'auto'>(initialSettings.noteHistoryMode)
   const [editingNote, setEditingNote] = useState<{ note: Note; readOnly?: boolean } | null>(null)
   const [isLoading, setIsLoading] = useState(false)