Ensure app returns to slideshow when entering dream mode

[brad]

When the app is opened in dream/screensaver mode, it should always open straight to the photos. This change ensures that if the app was left in the settings screen, it will automatically return to the slideshow when triggered as a screensaver.

Key changes:

• Modified _checkDreamMode() in lib/ui/screens/slideshow_screen.dart to use Navigator.of(context).popUntil((route) => route.isFirst) when isDream is true.
• This logic is executed both on initial launch and when the app is resumed (e.g. when the screensaver starts while the app was in the background or backgrounded by the user earlier).

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PR created automatically by Jules for task 10483135693755912570 started by @brad

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