Implement welcome screen and enhance session management in EditorManager

- Added a welcome screen that can be accessed from the View menu, improving user experience for new users.
- Enhanced session management by allowing the reuse of empty sessions when loading ROMs, reducing unnecessary session creation.
- Improved the session table layout for better readability and added a custom overworld feature flag for each session.
- Updated the DrawWelcomeScreen method for better visual integration and added a glow effect to the decorative line.

src/app/editor/editor_manager.h

@@ -156,6 +156,7 @@ class EditorManager {
   bool show_command_palette_ = false;
   bool show_global_search_ = false;
   bool show_session_rename_dialog_ = false;
+  bool show_welcome_screen_ = false;
   size_t session_to_rename_ = 0;
   char session_rename_buffer_[256] = {};