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/empty-paragraph-at-selection.ts

@@ -0,0 +1,19 @@
+import type { EditorState } from '@tiptap/pm/state'
+import type { Node as PMNode } from '@tiptap/pm/model'
+
+function isEmptyParagraph(node: PMNode): boolean {
+  if (node.type.name !== 'paragraph') return false
+  return node.textContent.trim() === '' && node.content.size <= 1
+}
+
+/** Paragraphe vide sous le curseur (critère US-3 Smart Paste). */
+export function getEmptyParagraphAtSelection(
+  state: EditorState,
+): { pos: number; node: PMNode } | null {
+  const { $from, empty } = state.selection
+  if (!empty) return null
+  if (!isEmptyParagraph($from.parent)) return null
+
+  const pos = $from.before($from.depth)
+  return { pos, node: $from.parent }
+}