Follow system light/dark theme in Android app

**Is your feature request related to a problem? Please describe.**
Many users today prefer their apps to automatically match the device’s global light or dark theme setting. Currently, the Wallabag Android app requires manual switching between light and dark modes, leading to an inconsistent user experience when the rest of the system UI changes automatically with time of day or battery settings.

**Describe the solution you’d like**
Introduce a “System default” option under **Settings → Appearance → Theme**, alongside “Light” and “Dark.” When selected, the app should use Android’s `DayNight` theming APIs (e.g. `AppCompatDelegate.MODE_NIGHT_FOLLOW_SYSTEM`) so that it automatically adapts whenever the user’s device theme changes.

**Describe alternatives you’ve considered**
- Manually toggling the in-app theme each time the system theme changes.
- Relying on third-party automation tools (e.g. Tasker) to switch the app’s settings — but these can be brittle and require extra setup.
- Leaving the app permanently in dark or light mode, which conflicts with the rest of the system when it switches.

**Additional context**
- Android’s Material Components library already provides a robust `DayNight` theming mechanism that many popular apps use by default.
- Users on Android 10+ frequently enable “Dark theme” at sunset or on low battery, and expect all of their apps to follow suit without manual intervention.
- This change would improve accessibility and reduce friction for users who rely on system-wide theme schedules.