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/components/MessageBubble.tsx

@@ -442,6 +442,32 @@ export function MessageBubble({
                   defaultValue: 'Nachricht nicht lesbar',
                 })}
               </span>
+            ) : parsed.kind === 'whiteboard' ? (
+              <div className="flex items-center gap-3 rounded-2xl border border-accent/30 bg-accent/5 px-4 py-3">
+                <div className="flex h-10 w-10 shrink-0 items-center justify-center rounded-lg bg-accent/20 text-accent">
+                  <svg viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth={2}
+                       strokeLinecap="round" strokeLinejoin="round" className="h-5 w-5">
+                    <rect x="3" y="4" width="18" height="13" rx="2" />
+                    <path d="M8 21h8M12 17v4" />
+                  </svg>
+                </div>
+                <div className="min-w-0 flex-1">
+                  <div className="text-sm font-semibold text-fg">Whiteboard</div>
+                  <div className="text-xs text-fg-muted">Gemeinsames Zeichnen</div>
+                </div>
+                <button
+                  type="button"
+                  onClick={() => {
+                    window.dispatchEvent(
+                      new CustomEvent('chatapp:open-whiteboard', { detail: { id: parsed.whiteboardId } }),
+                    );
+                  }}
+                  disabled={!parsed.whiteboardId}
+                  className="cursor-pointer rounded-md bg-accent px-3 py-1.5 text-xs font-semibold text-accent-fg transition hover:bg-accent/90 disabled:cursor-not-allowed disabled:opacity-50"
+                >
+                  Öffnen
+                </button>
+              </div>
             ) : parsed.kind === 'poll' ? (
               <PollCard
                 question={parsed.question}