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>

api_client.py

@@ -57,11 +57,17 @@ class HTTPAPIClient:
         message: str,
         user_id: str = "default",
         show_thinking: bool = False,
+        image_path: str | None = None,
     ) -> AsyncIterator[tuple[str, str | None]]:
+        payload: dict = {"message": message, "user_id": user_id, "show_thinking": show_thinking}
+        if image_path:
+            import base64
+            with open(image_path, "rb") as f:
+                payload["image_base64"] = base64.b64encode(f.read()).decode()
         async with self._client.stream(
             "POST",
             f"{self._url}/chat",
-            json={"message": message, "user_id": user_id, "show_thinking": show_thinking},
+            json=payload,
             timeout=self._timeout,
         ) as response:
             response.raise_for_status()