feat: make Gemini model configurable + auto-list available models on error

- gemini_model is now a saved config field (default: gemini-2.0-flash)
- Config panel shows a text input to set the model name
- probarGemini() reads the configured model and on error calls ListModels
  to show exactly which model names are available for the account's API key
- GeminiIntentService and GeminiVisionService build the URL from config at runtime

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/Services/GeminiIntentService.php

@@ -10,11 +10,13 @@ use Illuminate\Support\Facades\Log;
 class GeminiIntentService
 {
     private string $apiKey;
-    private string $apiUrl = 'https://generativelanguage.googleapis.com/v1beta/models/gemini-2.5-flash:generateContent';
+    private string $apiUrl;
 
     public function __construct()
     {
         $this->apiKey = ChatConfig::get('gemini_api_key', '');
+        $model        = ChatConfig::get('gemini_model', 'gemini-2.0-flash');
+        $this->apiUrl = "https://generativelanguage.googleapis.com/v1beta/models/{$model}:generateContent";
     }
 
     /**