Add DM typing indicator to the game chat

The server broadcasts a "typing" event to the whole room right when
it starts the DM turn, and broadcasts turn failures to everyone too
(previously only the sender saw them, which would've left other
players' indicators stuck forever). The frontend shows a pulsing
"Dungeon Master schreibt …" line that clears once the real reply or
an error arrives.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/components/TranscriptPane.tsx

@@ -9,12 +9,18 @@ function senderLabel(message: Message): string {
   return message.player_name ?? "Spieler";
 }
 
-export default function TranscriptPane({ messages }: { messages: Message[] }) {
+export default function TranscriptPane({
+  messages,
+  typing,
+}: {
+  messages: Message[];
+  typing?: boolean;
+}) {
   const bottomRef = useRef<HTMLDivElement>(null);
 
   useEffect(() => {
     bottomRef.current?.scrollIntoView({ block: "end" });
-  }, [messages]);
+  }, [messages, typing]);
 
   return (
     <div className="flex h-full flex-col gap-3 overflow-y-auto p-4">
@@ -34,6 +40,11 @@ export default function TranscriptPane({ messages }: { messages: Message[] }) {
           <span className="whitespace-pre-wrap">{m.content}</span>
         </div>
       ))}
+      {typing && (
+        <div className="text-slate-100">
+          <span className="animate-pulse font-semibold text-amber-400">Dungeon Master schreibt …</span>
+        </div>
+      )}
       <div ref={bottomRef} />
     </div>
   );