refactor: Update RecentFilesManager and ThemeManager for improved utility usage

- Refactored RecentFilesManager to utilize the utility function for ensuring the configuration directory exists, enhancing code clarity.
- Updated ThemeManager to use the utility function for retrieving file names, improving consistency across the codebase.
- Adjusted selected_editor initialization in EditorSelectionDialog to use static_cast for better type safety.

src/app/editor/ui/editor_selection_dialog.h

@@ -89,7 +89,7 @@ class EditorSelectionDialog {
   void DrawQuickAccessButtons();
   
   std::vector<EditorInfo> editors_;
-  EditorType selected_editor_ = EditorType::kNone;
+  EditorType selected_editor_ = static_cast<EditorType>(0);
   bool is_open_ = false;
   std::function<void(EditorType)> selection_callback_;