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/app/globals.css

@@ -3000,4 +3000,37 @@ html.font-system * {
   border-radius: 2px;
   box-shadow: 0 0 0 2px rgba(249, 115, 22, 0.3);
   color: inherit;
-}
+}
+
+/* Multi-column layout — seamless like Notion, no boxes */
+.tiptap-columns {
+  display: flex !important;
+  gap: 24px !important;
+  margin: 8px 0 !important;
+  padding: 0 !important;
+  border: none !important;
+  background: none !important;
+  border-radius: 0 !important;
+}
+
+.tiptap-column {
+  flex: 1 1 0% !important;
+  min-width: 0 !important;
+  padding: 0 !important;
+  border: none !important;
+  border-radius: 0 !important;
+  background: none !important;
+  border-left: 1px solid color-mix(in oklab, var(--foreground) 8%, transparent) !important;
+  padding-left: 24px !important;
+  overflow-wrap: break-word !important;
+  word-break: break-word !important;
+}
+
+.tiptap-column:first-child {
+  border-left: none !important;
+  padding-left: 0 !important;
+}
+
+.tiptap-column > *:first-child { margin-top: 0; }
+.tiptap-column > *:last-child { margin-bottom: 0; }
+.tiptap-column p { margin: 4px 0; }