- Add tauri-plugin-updater for app auto-updates
- Create Settings/Updater page with channel selection (Stable/Pre-Release)
- Bump version to 1.2.1 in Cargo.toml and tauri.conf.json
- Remove dummy data from Proxmox RemotesPage
- Collapse Proxmox section by default in sidebar
- Add updater backend commands: check_app_updates, install_app_updates
- Update plan document with implementation details
