feat: editor improvements and architectural grid prototype

Multiple feature additions and improvements across the application:

- NextGen Editor: drag handles, smart paste, block actions
- Structured views: Kanban and table layouts for notes
- Architectural Grid: new brainstorming/agent interface prototype
- Flashcards: SM-2 revision algorithm with AI generation
- MCP server: robustness improvements
- Graph/PDF chat: fix click propagation and copy behavior
- Various UI/UX enhancements and bug fixes

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

memento-note/components/editor-block-drag-handle.tsx

@@ -0,0 +1,78 @@
+'use client'
+
+/**
+ * Poignée drag globale (Novel / tiptap-extension-global-drag-handle).
+ * L’extension TipTap positionne cet élément ; le clic ouvre le menu bloc.
+ */
+import { memo, useEffect, useRef } from 'react'
+import type { Editor } from '@tiptap/core'
+import { BLOCK_DRAG_HANDLE_ID, resolveBlockAtDragHandle } from '@/lib/editor/block-at-drag-handle'
+
+type EditorBlockDragHandleProps = {
+  editor: Editor | null
+  onOpenMenu: (anchorRect: DOMRect) => void
+}
+
+export const EditorBlockDragHandle = memo(function EditorBlockDragHandle({
+  editor,
+  onOpenMenu,
+}: EditorBlockDragHandleProps) {
+  const dragStartedRef = useRef(false)
+  const onOpenMenuRef = useRef(onOpenMenu)
+  onOpenMenuRef.current = onOpenMenu
+
+  useEffect(() => {
+    if (!editor || editor.isDestroyed) return
+    const el = document.getElementById(BLOCK_DRAG_HANDLE_ID)
+    if (!el) return
+
+    const onPointerDown = () => {
+      dragStartedRef.current = false
+    }
+
+    const onDragStart = () => {
+      dragStartedRef.current = true
+    }
+
+    const onClick = (e: MouseEvent) => {
+      if (dragStartedRef.current) {
+        dragStartedRef.current = false
+        return
+      }
+      e.preventDefault()
+      e.stopPropagation()
+      const block = resolveBlockAtDragHandle(editor)
+      if (!block) return
+      onOpenMenuRef.current(el.getBoundingClientRect())
+    }
+
+    el.addEventListener('pointerdown', onPointerDown)
+    el.addEventListener('dragstart', onDragStart)
+    el.addEventListener('click', onClick)
+
+    return () => {
+      el.removeEventListener('pointerdown', onPointerDown)
+      el.removeEventListener('dragstart', onDragStart)
+      el.removeEventListener('click', onClick)
+    }
+  }, [editor])
+
+  return (
+    <div
+      id={BLOCK_DRAG_HANDLE_ID}
+      className="drag-handle hide"
+      role="button"
+      tabIndex={-1}
+      aria-hidden="true"
+    >
+      <svg width="14" height="14" viewBox="0 0 14 14" fill="currentColor" aria-hidden="true">
+        <circle cx="4" cy="3" r="1.2" />
+        <circle cx="10" cy="3" r="1.2" />
+        <circle cx="4" cy="7" r="1.2" />
+        <circle cx="10" cy="7" r="1.2" />
+        <circle cx="4" cy="11" r="1.2" />
+        <circle cx="10" cy="11" r="1.2" />
+      </svg>
+    </div>
+  )
+})