Fix model not supported on Novita; fix Gradio 6 theme deprecation warning

- Switch model from Llama-3.1-70B-Instruct (not on Novita) to
Llama-3.3-70B-Instruct (confirmed on Novita; newer/better model)
- Move theme= from gr.Blocks() to demo.launch() per Gradio 6 API
(gr.blocks.py:873 confirms launch() accepts theme param)

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

app.py

@@ -459,7 +459,7 @@ def respond(message, history):
     for attempt in range(3):
         try:
             response = client.chat_completion(
-                model="meta-llama/Llama-3.1-70B-Instruct",
+                model="meta-llama/Llama-3.3-70B-Instruct",  # 3.1-70B not on Novita; 3.3-70B is confirmed available
                 messages=messages,
                 max_tokens=512,
             )
@@ -507,7 +507,7 @@ textbox = gr.Textbox(
     scale=7,
 )
 
-with gr.Blocks(theme=theme, title="Sanyu — Peer Support Chatbot") as demo:
+with gr.Blocks(title="Sanyu — Peer Support Chatbot") as demo:  # theme moved to launch() per Gradio 6
     gr.ChatInterface(
         fn=respond,
         chatbot=chatbot,
@@ -533,4 +533,4 @@ with gr.Blocks(theme=theme, title="Sanyu — Peer Support Chatbot") as demo:
         cache_examples=False,
     )
 
-demo.launch()
+demo.launch(theme=theme)