Add launch at login setting

[olerida]

Summary

Allow users to configure whether Settle launches automatically when they log in to macOS.

Acceptance criteria

• Add a Launch at Login toggle under Settings.
• Keep the toggle synchronized with the actual macOS login-item state.
• Persist and handle state changes reliably.
• Localize all user-facing text in English, Spanish, Catalan, French, and German.

Dependencies

• Depends on Implement Settings #30.

Related work

• Add restore at login with explicit default layout selection #6 covers restoring a selected layout when the session starts.