Implement SaveRomAs functionality and enhance session management

- Added SaveRomAs method to allow users to save ROM files with specified filenames, ensuring proper file extensions are applied.
- Updated the DrawMenuBar method to utilize SaveRomAs, providing feedback on save success or failure through toast notifications.
- Enhanced session management by introducing RemoveSession method to mark sessions as closed instead of removing them, improving user experience when managing active sessions.
- Updated session handling logic to ensure closed sessions are properly skipped in the UI, maintaining clarity in session management.

src/app/editor/editor_manager.h

@@ -117,6 +117,7 @@ class EditorManager {
   absl::Status LoadRom();
   absl::Status LoadAssets();
   absl::Status SaveRom();
+  absl::Status SaveRomAs(const std::string& filename);
   absl::Status OpenRomOrProject(const std::string& filename);
   
   // Enhanced project management
@@ -220,8 +221,10 @@ class EditorManager {
   void CreateNewSession();
   void DuplicateCurrentSession();
   void CloseCurrentSession();
+  void RemoveSession(size_t index);
   void SwitchToSession(size_t index);
   size_t GetCurrentSessionIndex() const;
+  size_t GetActiveSessionCount() const;
   void ResetWorkspaceLayout();
   
   // Multi-session editor management