fix: resolve three travel agent UI issues

- Connection error: wrap sync get_llm_api_key() in sync_to_async in
  stream_chat_completion() to fix SynchronousOnlyOperation raised when
  the async SSE generator calls a synchronous Django ORM function
- Models not loading: add opencode_zen handler to models endpoint
  returning its default model; fix frontend to show 'Default' instead
  of 'Loading...' indefinitely when no model list is returned
- Location in header: remove destination subtitle from Travel Assistant
  header — collection-wide chat has no single meaningful location

backend/server/chat/views/__init__.py

@@ -414,6 +414,9 @@ class ChatProviderCatalogViewSet(viewsets.ViewSet):
                     pass
                 return Response({"models": []})
 
+            if provider in ["opencode_zen"]:
+                return Response({"models": ["openai/gpt-5-nano"]})
+
             return Response({"models": []})
         except Exception as exc:
             logger.error("Failed to fetch models for %s: %s", provider, exc)