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/lib/types.ts

@@ -13,12 +13,13 @@ export interface Notebook {
   icon: string | null;
   color: string | null;
   order: number;
+  parentId: string | null;
   userId: string;
   createdAt: Date;
   updatedAt: Date;
-  // Relations
   notes?: Note[];
   labels?: Label[];
+  children?: Notebook[];
 }
 
 /**