[Feature]: Add rename, delete, and duplicate actions for project items #78
Description
Problem
Basic file management still requires leaving the editor for common project-tree actions.
Proposed solution
Add rename, delete, and duplicate actions for files and folders in the project sidebar, with platform-appropriate confirmation for destructive steps.
Acceptance criteria
- Files can be renamed from the project sidebar
- Folders can be renamed from the project sidebar
- Files can be duplicated from the project sidebar
- Delete actions require confirmation where destructive
- Sidebar state refreshes cleanly after changes
- Error states are user-visible without breaking selection or focus order