Exit app on center tap in dream mode

[brad]

This PR implements a change to the center tap behavior when the app is running in Android Screen Saver (Dream) mode. Instead of always opening the settings page, a tap in the center of the screen will now exit the app and return the user to the previously open app or home screen.

Key changes:

• Native Android: Modified PhotoFrameDreamService to include an is_dream flag when launching MainActivity.
• Native Android: Updated MainActivity to track the isDreamMode status and ScreenControlHandler to expose it to Flutter.
• Native Android: Added an exitApp method to ScreenControlHandler that uses finishAndRemoveTask() to cleanly exit.
• Flutter: Updated NativeScreenControlService to include isDreamMode() and exitApp() methods.
• Flutter UI: Updated SlideshowScreen to check for dream mode status on initialization and resumption, and conditionally exit on center tap.
• Localizations: Added 'Tap to exit' string in English and German for the "No photos found" state when in dream mode.

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PR created automatically by Jules for task 1538978416310908557 started by @brad

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