fix: stabilize ai design history switching

shared/image-workspace.ts

@@ -105,6 +105,13 @@ export type DesignConversationSummary = {
 
 export type DesignConversation = DesignConversationSummary & {
   messages: DesignMessage[];
+  /** Cursor for the next older message page; absent only for legacy adapters. */
+  messagePage?: DesignConversationMessagePage;
+};
+
+export type DesignConversationMessagePage = {
+  hasOlder: boolean;
+  nextBefore: string | null;
 };
 
 export type DesignAsset = {