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>

resources/views/livewire/chat/show-configuracion-chat.blade.php

@@ -69,6 +69,12 @@
                     <input wire:model.defer="gemini_api_key" type="password" placeholder="AIza..."
                         class="w-full border border-gray-300 rounded-lg px-3 py-2 text-sm focus:ring-2 focus:ring-emerald-500 outline-none font-mono">
                 </div>
+                <div>
+                    <label class="block text-sm font-semibold text-gray-700 mb-1">Modelo</label>
+                    <p class="text-xs text-gray-400 mb-2">Usa "Probar conexión" para ver los modelos disponibles en tu cuenta si no sabes cuál poner.</p>
+                    <input wire:model.defer="gemini_model" type="text" placeholder="gemini-2.0-flash"
+                        class="w-full border border-gray-300 rounded-lg px-3 py-2 text-sm focus:ring-2 focus:ring-emerald-500 outline-none font-mono">
+                </div>
                 <div class="flex items-center gap-3">
                     <input wire:model.defer="gemini_habilitado" type="checkbox" value="1"
                         id="gemini_hab" class="w-4 h-4 accent-emerald-600">