feat(P4B.T6): composer 'Whiteboard' button creates board + sends bubble

apps/desktop/src/lib/conversationFeatures.ts

@@ -4,6 +4,7 @@ import {
   type AttachmentHandle,
   type DecryptedMessage,
   type PollOption,
+  type WhiteboardPayload,
 } from '@chat-app/shared/chat';
 
 export type AttachmentBucket = 'media' | 'audio' | 'files';
@@ -97,6 +98,15 @@ export function createPollPayload(question: string, optionTexts: string[]): stri
   });
 }
 
+export function createWhiteboardPayload(whiteboardId: string): string {
+  const payload: WhiteboardPayload = {
+    v: 1,
+    type: 'whiteboard',
+    whiteboard_id: whiteboardId,
+  };
+  return serializeMessagePayload(payload);
+}
+
 export function summarizePollVotes(
   options: PollOption[],
   reactions: ReactionSummaryInput[],