fix: Handle empty assistant responses when visualizations are generated

When segmentation/grounding tools run, Gemini sometimes returns empty
responses, causing blank chat bubbles in the UI.

Solution: Provide helpful default messages when:
- Segmentation visualization is created
- Grounding visualization is created
- Any other empty response occurs

This ensures users always see a message explaining what happened.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

app.py

@@ -254,6 +254,17 @@ def chat(message, history, mode):
         if os.path.exists(latest_viz):
             viz_image = latest_viz
 
+            # If assistant message is empty but we have a visualization, provide a default message
+            if not assistant_message or assistant_message.strip() == "":
+                if "segmentation" in latest_viz:
+                    assistant_message = "I've segmented the requested anatomical structures. The visualization is shown on the right."
+                elif "grounding" in latest_viz:
+                    assistant_message = "I've highlighted the requested regions. The visualization is shown on the right."
+
+    # Final fallback for empty messages
+    if not assistant_message or assistant_message.strip() == "":
+        assistant_message = "I processed your request. Please check the visualization panel on the right."
+
     return assistant_message, viz_image
 
 # Custom interface with image output