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/prisma/schema.prisma

@@ -82,6 +82,7 @@ model Notebook {
   icon          String?
   color         String?
   order         Int
+  parentId      String?
   userId        String
   createdAt     DateTime       @default(now())
   updatedAt     DateTime       @updatedAt
@@ -91,9 +92,12 @@ model Notebook {
   notes         Note[]
   user          User           @relation(fields: [userId], references: [id], onDelete: Cascade)
   workflows     Workflow[]
+  parent        Notebook?      @relation("NotebookTree", fields: [parentId], references: [id], onDelete: Cascade)
+  children      Notebook[]     @relation("NotebookTree")
 
   @@index([userId, order])
   @@index([userId])
+  @@index([parentId])
 }
 
 model Label {