[devy1540]

Closes #1455

Description

Adds a System option to the mobile app's theme settings. The app follows the device's light or dark appearance while the picker continues to show System as the selected preference.

• Persist Light, Dark, or System in the existing MMKV-backed preferences store and restore the selection in its own guarded layout effect, keeping locale and splash initialization limited to startup.
• Delegate appearance changes to the existing NativeWind API, keeping the effective color scheme as Light or Dark for existing consumers.
• Preserve explicit EPUB reader themes while allowing the default reader theme to follow the app's appearance. Update Android navigation-bar styling from the same background state used by SystemBars.
• Add English and Korean labels for the new option.
• Add 11 focused hook tests using small mocks in the existing Node test environment, without adding a jsdom dependency to Expo.

Verification

• Expo test suite: 65 tests passed, including the 11 focused theme-hook tests.
• ESLint and Prettier checks passed for the changed TypeScript files and package manifest.
• Production Hermes bundle exports succeeded for both iOS and Android.
• Full Expo type checking reports the same 15 errors as the unchanged nightly base (7752c39); this change adds no diagnostics.
• The theme tests mock NativeWind and the preference-store hooks. Native device or simulator UI verification is still pending.

AI assistance

Codex assisted with the implementation and review. The verification checks listed above were run locally.

Ready?

• I read the contributing guidelines.
• I searched for related issues and pull requests.
• This PR targets nightly.
• I added tests for the changed behavior.

Stump Contributor License Agreement

These contributions are licensed under GPL-3.0 for the Expo application and MIT for the shared translation files, in accordance with the repository's contributing terms.

[aaronleopold]

Thank you!