Add Settings About page

[yvette-carlisle]

Summary

• Add a native Settings About section with Yvette Cipher intro copy.
• Link the About page to the Rsnap GitHub repo and @YvetteCipher on X.
• Document the About Settings contract and keep Restore Defaults off non-configuration sections.

Validation

• cargo make lint-swift
• cargo make test-macos-native-host
• cargo make test-macos-native-host-stage
• cargo make checks