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/lib/editor/global-drag-handle-extension.ts

@@ -0,0 +1,17 @@
+import GlobalDragHandle from 'tiptap-extension-global-drag-handle'
+import AutoJoiner from 'tiptap-extension-auto-joiner'
+import { BLOCK_DRAG_HANDLE_ID, BLOCK_DRAG_HANDLE_WIDTH } from './block-at-drag-handle'
+
+/** Extensions drag handle Novel / Notion — une poignée globale, position fixed */
+export const globalDragHandleExtensions = [
+  GlobalDragHandle.configure({
+    dragHandleWidth: BLOCK_DRAG_HANDLE_WIDTH,
+    scrollTreshold: 100,
+    dragHandleSelector: `#${BLOCK_DRAG_HANDLE_ID}`,
+    excludedTags: [],
+    customNodes: [],
+  }),
+  AutoJoiner.configure({
+    elementsToJoin: ['bulletList', 'orderedList'],
+  }),
+]