Phase 17: Add image upload to chat UI

- app.py: image_input gr.Image component, respond() accepts image_path,
  all yields updated to 7 outputs
- api_client.py: chat(image_path=None), base64-encodes image for API
- services/chat.py: chat(image_path=None) passes through to api_client

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

services/chat.py

@@ -8,9 +8,13 @@ class ChatService:
         self._api = api_client
 
     def chat(
-        self, message: str, user_id: str, show_thinking: bool
+        self,
+        message: str,
+        user_id: str,
+        show_thinking: bool,
+        image_path: str | None = None,
     ) -> AsyncIterator[tuple[str, str | None]]:
-        return self._api.chat(message, user_id, show_thinking)
+        return self._api.chat(message, user_id, show_thinking, image_path=image_path)
 
     async def reset(self, user_id: str) -> None:
         await self._api.reset(user_id)