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/context/notebooks-context.tsx

@@ -14,6 +14,7 @@ export interface CreateNotebookInput {
   name: string
   icon?: string
   color?: string
+  parentId?: string | null
 }
 
 export interface UpdateNotebookInput {