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/locales/en.json

@@ -2445,6 +2445,12 @@
     "mathCancel": "Cancel",
     "mathAi": "Write with AI",
     "mathAiPlaceholder": "Describe the equation in words... (e.g: quadratic formula)",
+    "slashColumns": "Columns",
+    "slashColumnsDesc": "Put content side by side",
+    "columnsRemove": "Remove a column",
+    "columnsAdd": "Add a column",
+    "columnsDelete": "Delete columns",
+    "columnsLabel": "columns",
     "calloutDelete": "Delete callout",
     "calloutUnwrap": "Disable callout",
     "calloutInfo": "Information",