feat: colonnes multi-colonnes (layout côte à côte style Notion)

- Architecture nested nodes: columns container → column children
- CSS seamless (pas de bordures, fin séparateur vertical entre colonnes)
- isolating: true sur les deux nœuds (curseur reste dans sa colonne)
- Commands addColumnBefore/addColumnAfter/deleteColumn
- Slash menu + drag handle + raccourci Mod+Shift+L
- i18n FR/EN complet

memento-note/components/block-action-menu.tsx

@@ -14,12 +14,14 @@ import {
   Heading1, Heading2, Heading3, List, ListOrdered,
   CheckSquare, Quote, CodeXml, Database,
   ArrowUp, ArrowDown, AlignLeft, ClipboardCopy, Sparkles,
-  ChevronsRightLeft, MessageSquareWarning, ListTree,
+  ChevronsRightLeft, MessageSquareWarning, ListTree, Columns3,
+  Plus, Minus,
 } from 'lucide-react'
 import { replaceBlockWithStructuredView } from '@/components/tiptap-structured-view-block-extension'
 import { insertToggleBlock, turnIntoToggleBlock } from '@/components/tiptap-toggle-extension'
 import { turnIntoCalloutBlock } from '@/components/tiptap-callout-extension'
 import { insertOutlineBlock } from '@/components/tiptap-outline-extension'
+import { insertColumnsBlock } from '@/components/tiptap-columns-extension'
 
 interface BlockActionMenuProps {
   editor: Editor
@@ -391,6 +393,12 @@ export function BlockActionMenu({
         <span>{t('richTextEditor.slashOutline')}</span>
       </button>
 
+      {/* Colonnes */}
+      <button type="button" className="block-action-item" onClick={() => { insertColumnsBlock(editor, 2); onClose() }}>
+        <Columns3 size={16} />
+        <span>{t('richTextEditor.slashColumns')}</span>
+      </button>
+
       <div className="block-action-separator" />
 
       {/* Création de diagramme */}