[Feature]: Add new file and new folder actions to the project sidebar #79
Description
Problem
The project sidebar can open and browse files, but it does not yet cover basic file creation flows directly where users are already working.
Proposed solution
Add lightweight project-tree actions for creating a new file and a new folder from the sidebar context menu and relevant toolbar entry points.
Acceptance criteria
- New File is available from the project sidebar
- New Folder is available from the project sidebar
- Creation happens in the selected folder or project root
- The project tree refreshes and reveals the new item
- Keyboard and VoiceOver labels remain clear on macOS and iPad keyboard workflows