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/hooks/use-note-drag.ts

@@ -1,36 +0,0 @@
-'use client'
-
-import { useState, useCallback } from 'react'
-
-export type DragState = 'idle' | 'dragging' | 'drag-over'
-
-export function useNoteDrag() {
-  const [draggedNoteId, setDraggedNoteId] = useState<string | null>(null)
-  const [dragOverNotebookId, setDragOverNotebookId] = useState<string | null>(null)
-
-  const startDrag = useCallback((noteId: string) => {
-    setDraggedNoteId(noteId)
-  }, [])
-
-  const endDrag = useCallback(() => {
-    setDraggedNoteId(null)
-    setDragOverNotebookId(null)
-  }, [])
-
-  const dragOver = useCallback((notebookId: string | null) => {
-    setDragOverNotebookId(notebookId)
-  }, [])
-
-  const isDragging = draggedNoteId !== null
-  const isDragOver = dragOverNotebookId !== null
-
-  return {
-    draggedNoteId,
-    dragOverNotebookId,
-    startDrag,
-    endDrag,
-    dragOver,
-    isDragging,
-    isDragOver,
-  }
-}

memento-note/hooks/use-resize-observer.ts

@@ -1,33 +0,0 @@
-'use client';
-
-import { useEffect, useRef } from 'react';
-
-export function useResizeObserver(callback: (entry: ResizeObserverEntry) => void) {
-  const ref = useRef<HTMLElement>(null);
-  const frameId = useRef<number>(0);
-
-  useEffect(() => {
-    const element = ref.current;
-    if (!element) return;
-
-    const observer = new ResizeObserver((entries) => {
-      // Cancel previous frame to avoid stacking updates
-      if (frameId.current) cancelAnimationFrame(frameId.current);
-      
-      frameId.current = requestAnimationFrame(() => {
-        for (const entry of entries) {
-          callback(entry);
-        }
-      });
-    });
-
-    observer.observe(element);
-
-    return () => {
-      observer.disconnect();
-      if (frameId.current) cancelAnimationFrame(frameId.current);
-    };
-  }, [callback]);
-
-  return ref;
-}