Support both phoenix and mixer engines in chat

app.py

@@ -366,9 +366,10 @@ def phox_chat(message, history):
         return "Engine not initialized. Please wait for startup."
 
     with _state_lock:
-        phoenix_running = _phoenix_proc is not None and _phoenix_proc.poll() is None
-    if not phoenix_running:
-        return "Phoenix is not running. Please click 'Start Engine' first."
+        engine_running = (_phoenix_proc is not None and _phoenix_proc.poll() is None) or \
+                         (_mixer_proc is not None and _mixer_proc.poll() is None)
+    if not engine_running:
+        return "Engine not running. Please click 'Start Engine' or 'Start Mixer' first."
 
     if _tokenizer is None:
         return "Tokenizer not loaded."