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/app/(main)/page.tsx

@@ -13,9 +13,7 @@ export default async function HomePage() {
       ? ('masonry' as const)
       : settings?.notesViewMode === 'tabs'
         ? ('tabs' as const)
-        : settings?.notesViewMode === 'list'
-          ? ('list' as const)
-          : ('masonry' as const)
+        : ('masonry' as const)
 
   return (
     <HomeClient