Move offline reading settings from JSON-based UserDefaults storage to CoreData SettingEntity for consistency with other app settings.
Changes:
- Add offline settings fields to SettingEntity (offlineEnabled, offlineMaxUnreadArticles, offlineSaveImages, offlineLastSyncDate)
- Update SettingsRepository to use CoreData instead of UserDefaults for offline settings
- Use consistent async/await pattern with withCheckedThrowingContinuation
- Remove JSON encoding/decoding in favor of direct CoreData properties
Benefits:
- Unified settings storage in CoreData
- Type-safe property access
- Automatic migration support
- No redundant UserDefaults entries