Add default AI model selection to settings

Replaces the previous placeholder for default LLM selection with a working default AI model selector in settings. Updates SettingsModel and related providers to support storing and updating the default AI model as a JSON object. Integrates the selector UI and ensures new AI requests use the default model if set.

lib/screens/common_widgets/code_pane.dart

@@ -29,7 +29,8 @@ class CodePane extends ConsumerWidget {
         ? getRequestModelFromHistoryModel(selectedHistoryRequestModel!)
         : ref.watch(selectedRequestModelProvider);
 
-    if (selectedRequestModel!.apiType == APIType.ai) {
+    // TODO: Add AI Request Codegen
+    if (selectedRequestModel?.apiType == APIType.ai) {
       return const ErrorMessage(
         message: "Code generation for AI Requests is currently not available.",
       );