Implement launcher update check and improve migration process

[alex73630]

This pull request introduces several improvements and new features to the Peacock AppImage launcher, focusing on enhancing the migration process from the legacy setup, adding update checking and download functionality for the launcher itself, and improving service management during migration. It also updates documentation to guide users through these new workflows.

Migration and Service Management Improvements:

• The migration process now replaces any previous service definition and restores the prior enabled/running state, with detailed tracking of what was replaced or restored in a new MigrationResult structure. This ensures smoother transitions from legacy setups and clearer feedback to the user. [1] [2] [3] [4] [5]

Launcher Update Detection and Download:

• Adds a new LauncherStatus type and logic to check for updates by querying the latest GitHub release, displaying "Download latest launcher" in the main menu, and opening the releases page in the browser when selected. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

Documentation Updates:

• Adds a comprehensive MIGRATION.md guide and updates the README.md to document the migration process, new service management, and the recommended workflow for switching from the legacy patcher method. [1] [2] [3]
• The README now includes instructions for enabling Peacock on boot and clarifies the new migration workflow. [1] [2]

Build and Versioning Enhancements:

• The CI workflow now sets the launcher version in Cargo.toml from the git tag at build time, ensuring accurate version reporting for update checks. [1] [2]

Minor Code and Usability Improvements:

• Various small code cleanups and formatting improvements for better maintainability. [1] [2] [3]

These changes collectively modernize the launcher experience, make migrations safer and more transparent, and introduce a user-friendly update mechanism for the AppImage itself.