Restrict free tier: disable custom model input and hide model_source

- Set allow_custom_value=False for free tier model dropdown
- Hide model_source dropdown for free tier (only BYOK needs it)
- Toggle both settings correctly when switching between tiers

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

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

app.py

@@ -1373,12 +1373,14 @@ Soria, C. (2025). CatLLM: A Python package for LLM-based text classification. DO
                     choices=FREE_MODEL_CHOICES,
                     value="Qwen/Qwen3-VL-235B-A22B-Instruct:novita",
                     label="Model",
-                    allow_custom_value=True
+                    allow_custom_value=False,  # Only allow custom for "Bring Your Own Key"
+                    interactive=True
                 )
                 model_source = gr.Dropdown(
                     choices=["auto", "openai", "anthropic", "google", "mistral", "xai", "huggingface", "perplexity"],
                     value="auto",
-                    label="Model Source"
+                    label="Model Source",
+                    visible=False  # Hide for free tier, show for BYOK
                 )
                 api_key = gr.Textbox(
                     label="API Key",
@@ -1431,21 +1433,23 @@ Soria, C. (2025). CatLLM: A Python package for LLM-based text classification. DO
     def update_model_tier(tier):
         if tier == "Free Models":
             return (
-                gr.update(choices=FREE_MODEL_CHOICES, value=FREE_MODEL_CHOICES[0]),
-                gr.update(visible=False),
+                gr.update(choices=FREE_MODEL_CHOICES, value=FREE_MODEL_CHOICES[0], allow_custom_value=False),
+                gr.update(visible=False),  # model_source hidden for free
+                gr.update(visible=False),  # api_key hidden for free
                 "**Free tier** - no API key required!"
             )
         else:
             return (
-                gr.update(choices=PAID_MODEL_CHOICES, value=PAID_MODEL_CHOICES[0]),
-                gr.update(visible=True),
+                gr.update(choices=PAID_MODEL_CHOICES, value=PAID_MODEL_CHOICES[0], allow_custom_value=True),
+                gr.update(visible=True),   # model_source shown for BYOK
+                gr.update(visible=True),   # api_key shown for BYOK
                 "**Bring Your Own Key** - enter your API key below."
             )
 
     model_tier.change(
         fn=update_model_tier,
         inputs=[model_tier],
-        outputs=[model, api_key, api_key_status]
+        outputs=[model, model_source, api_key, api_key_status]
     )
 
     spreadsheet_file.change(