feat(P4B.T7): MessageBubble renders whiteboard kind with Öffnen button

Adds a whiteboard branch to the MessageBubble ternary chain that renders
a card with an icon, label, and Öffnen button dispatching the
chatapp:open-whiteboard CustomEvent. ConversationPage now listens for
that event via a useEffect and calls setOpenWhiteboardId to open the
modal.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/desktop/src/pages/ConversationPage.tsx

@@ -463,6 +463,15 @@ export function ConversationPage() {
     };
   }, [id, setActiveConversation]);
 
+  useEffect(() => {
+    const onOpen = (e: Event) => {
+      const detail = (e as CustomEvent<{ id?: string }>).detail;
+      if (detail?.id) setOpenWhiteboardId(detail.id);
+    };
+    window.addEventListener('chatapp:open-whiteboard', onOpen);
+    return () => window.removeEventListener('chatapp:open-whiteboard', onOpen);
+  }, []);
+
   useEffect(() => {
     if (id && messages.length > 0) markRead(id);
   }, [id, messages.length, markRead]);