This change introduces Git stash support end-to-end, including a new
backend command to list stashes and operations to push, apply, pop, and
drop them. The frontend now fetches stashes as part of the repository
bundle and provides a dedicated panel to manage shelved changes.
- Add GitStash model and Tauri commands for stash operations
- Implement StashPanel component and wire it into the app
- Adjust sidebar layout and add stash-specific styling
