Fix API endpoint to use relative URL instead of localhost

- Changed fetch URL from 'http://localhost:8000/chat/stream' to '/chat/stream'
- Fixes chat functionality on HF Spaces deployment

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

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

test_interface.html

@@ -267,7 +267,7 @@
 
             try {
                 // Stream response
-                const response = await fetch('http://localhost:8000/chat/stream', {
+                const response = await fetch('/chat/stream', {
                     method: 'POST',
                     headers: {
                         'Content-Type': 'application/json',