Add application feedback and refine headless canvas design boundaries

src/lib/notifications.ts

@@ -0,0 +1,10 @@
+"use client";
+
+import { toast } from "sonner";
+
+/** Application adapter. Domain/editor packages must not import Sonner. */
+export type Notification = { id: string; title: string; description?: string; level?: "info" | "success" | "warning" | "error" };
+export function notify({ id, title, description, level = "info" }: Notification) {
+  return toast[level](title, { id, description });
+}
+export function dismissNotification(id: string) { toast.dismiss(id); }