fix: localize backup section labels

[pixel-daemon-bot]

Summary

This PR makes BackupSection labels and descriptions localizable by moving their presentation text from hardcoded strings in the data model to Android string resources.

Changes

• add @StringRes labelRes and descriptionRes to BackupSection;
• preserve key, iconRes, and sinceVersion unchanged;
• update Backup UI presentation sites to resolve the resource IDs at the UI boundary;
• add English and Russian resources for all current BackupSection labels and descriptions;
• update BackupSectionTest for the current 12-section model and validate resource parity.

Architecture

The stable BackupSection.key remains the persistence/serialization identity.

Presentation text is now represented by labelRes and descriptionRes, following existing PixelPlayer patterns for resource IDs in models.

The older label / description String fields remain temporarily as a compatibility bridge because Backup/Restore progress and validation localization are intentionally deferred to subsequent changes.

No Android Context was introduced into the data layer.

Validation

Passed locally:

• BackupSectionTest — 12/12
• :app:processDebugResources
• :app:compileDebugKotlin
• git diff --check

Scope

This PR intentionally covers only BackupSection presentation localization.

Backup/Restore progress events, validation error messages, and the broader localization system are outside this PR.

Do not claim that Backup/Restore is fully localized.