feat: Update AI Model References and Enhance Agent Chat Widget Functionality

- Updated AI model references from "gemini-2.0-flash-exp" to "gemini-2.5-flash" across various components, ensuring compatibility with the latest AI capabilities.
- Enhanced the AgentChatWidget to improve user experience with new features such as message copying, improved input handling, and session management.
- Introduced a chat session management system, allowing users to create and switch between multiple chat sessions seamlessly.
- Improved UI elements for better visibility and interaction, including styled buttons and tooltips for enhanced usability.

src/app/editor/editor_manager.cc

@@ -301,7 +301,7 @@ void EditorManager::Initialize(const std::string& filename) {
     // Create Gemini service
     cli::GeminiConfig config;
     config.api_key = api_key;
-    config.model = "gemini-2.0-flash-exp";  // Use vision-capable model
+    config.model = "gemini-2.5-flash";  // Use vision-capable model
     config.verbose = false;
     
     cli::GeminiAIService gemini_service(config);
@@ -350,10 +350,18 @@ void EditorManager::Initialize(const std::string& filename) {
   
   // Initialize editor selection dialog callback
   editor_selection_dialog_.SetSelectionCallback([this](EditorType type) {
-    if (!current_editor_set_) return;
-    
     editor_selection_dialog_.MarkRecentlyUsed(type);
     
+    // Handle agent editor separately (doesn't require ROM)
+    if (type == EditorType::kAgent) {
+#ifdef YAZE_WITH_GRPC
+      agent_editor_.set_active(true);
+#endif
+      return;
+    }
+    
+    if (!current_editor_set_) return;
+    
     switch (type) {
       case EditorType::kOverworld:
         current_editor_set_->overworld_editor_.set_active(true);