fix(chat): simplify user label to German; expose host via env var

- Change user message author label from "Du / You" to "Du" (German-first)
- Add CHAINLIT_HOST env var to .env.example so the server listens on all
  interfaces and is reachable from outside localhost

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

chat_app.py

@@ -93,7 +93,7 @@ async def on_chat_start() -> None:
             len(state.messages),
         )
         for msg in state.messages:
-            author = "Spielgruppe Pumuckl" if msg.role == "assistant" else "Du / You"
+            author = "Spielgruppe Pumuckl" if msg.role == "assistant" else "Du"
             await cl.Message(content=msg.content, author=author).send()
         return