Add comprehensive Perplexity model selection in setup wizard

Setup wizard improvements:
- Expanded model dropdown with 6 Perplexity models:
  * sonar-pro (recommended default)
  * sonar
  * sonar-reasoning
  * llama-3.1-sonar-small-128k-online
  * llama-3.1-sonar-large-128k-online
  * llama-3.1-sonar-huge-128k-online

- Added info button (?) next to model selector
- Info popup explains each model's characteristics:
  * Performance
  * Use cases
  * Speed/quality tradeoffs
  * Cost considerations

Created MODEL_CHANGE.md documentation:
- How to change model in .env file
- How to re-run setup wizard
- Detailed model descriptions
- Example configuration

Users can now easily choose the right model for their needs
and change it anytime without reconfiguring everything.

gui.py

@@ -218,7 +218,18 @@ class SetupWizard:
 
             [sg.Text("PERPLEXITY CONFIGURATION", font=("Arial", 12, "bold"))],
             [sg.Text("API Key:", size=(20, 1)), sg.Input("pplx-", key="-API-KEY-", password_char="*")],
-            [sg.Text("Model:", size=(20, 1)), sg.Combo(["sonar-pro", "sonar"], default_value="sonar-pro", key="-MODEL-")],
+            [sg.Text("Model:", size=(20, 1)),
+             sg.Combo([
+                 "sonar-pro",
+                 "sonar",
+                 "sonar-reasoning",
+                 "llama-3.1-sonar-small-128k-online",
+                 "llama-3.1-sonar-large-128k-online",
+                 "llama-3.1-sonar-huge-128k-online"
+             ], default_value="sonar-pro", key="-MODEL-", size=(35, 1)),
+             sg.Button("?", key="-MODEL-INFO-", size=(2, 1))],
+            [sg.Text("", size=(20, 1)),
+             sg.Text("Recommended: sonar-pro (best quality)", font=("Arial", 8), text_color="gray")],
             [sg.Text("Max Tokens:", size=(20, 1)), sg.Input("450", key="-TOKENS-")],
 
             [sg.HorizontalSeparator()],
@@ -240,6 +251,43 @@ class SetupWizard:
             if event == sg.WINDOW_CLOSED or event == "-CANCEL-":
                 break
 
+            # Show model info
+            if event == "-MODEL-INFO-":
+                model_info = """PERPLEXITY MODEL OVERVIEW:
+
+🔵 sonar-pro (RECOMMENDED)
+   - Best quality and reasoning capabilities
+   - Ideal for chat bots and complex queries
+   - Good balance of speed and intelligence
+
+🟢 sonar
+   - Faster, lower cost
+   - Good for simple queries
+   - Less complex reasoning
+
+🧠 sonar-reasoning
+   - Enhanced reasoning for complex problems
+   - Slower but more thoughtful responses
+
+📚 llama-3.1-sonar-small-128k-online
+   - Smallest, fastest Llama model
+   - 128k context window
+   - Best for simple, quick responses
+
+📚 llama-3.1-sonar-large-128k-online
+   - Larger Llama model
+   - Better quality than small
+   - Good balance
+
+📚 llama-3.1-sonar-huge-128k-online
+   - Largest Llama model
+   - Best quality but slower
+   - Higher cost
+
+For most chat bots: Use sonar-pro"""
+                sg.popup_scrolled(model_info, title="Model Information", size=(60, 25))
+                continue
+
             if event == "-SAVE-":
                 # Validate inputs
                 if not values["-OAUTH-"].startswith("oauth:"):