Add keep screen on feature to FrontendScreen

[TimoPtr]

Summary

Add keep screen on feature to FrontendScreen by exposing a flow from the preference to react on changes.

Checklist

• New or updated tests have been added to cover the changes following the testing guidelines.
• The code follows the project's code style and best_practices.
• The changes have been thoroughly tested, and edge cases have been considered.
• Changes are backward compatible whenever feasible. Any breaking changes are documented in the changelog for users and/or in the code for developers depending on the relevance.

[Copilot]

Pull request overview

This PR wires the existing “Keep screen on” preference into the new Compose-based frontend by exposing it as a reactive Flow from PrefsRepository, surfacing it as a StateFlow in FrontendViewModel, and applying it in FrontendScreen by toggling View.keepScreenOn while the screen is in composition.

Changes:

• Added PrefsRepository.keepScreenOnFlow() and implemented it via LocalStorage.observeChanges(...) with immediate initial emission
• Exposed keepScreenOnEnabled: StateFlow<Boolean> on FrontendViewModel driven by the preference flow
• Applied the preference in FrontendScreen via a DisposableEffect that sets/clears View.keepScreenOn, plus unit tests for the new flows/state

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
common/src/main/kotlin/io/homeassistant/companion/android/common/data/prefs/PrefsRepository.kt	Adds public keepScreenOnFlow() API with KDoc describing emission semantics
common/src/main/kotlin/io/homeassistant/companion/android/common/data/prefs/PrefsRepositoryImpl.kt	Implements keepScreenOnFlow() using LocalStorage.observeChanges
common/src/test/kotlin/io/homeassistant/companion/android/common/data/prefs/PrefsRepositoryImplTest.kt	Adds unit test verifying keepScreenOnFlow() emits initial + updated values on key changes
app/src/main/kotlin/io/homeassistant/companion/android/frontend/FrontendViewModel.kt	Adds keepScreenOnEnabled StateFlow for UI consumption
app/src/test/kotlin/io/homeassistant/companion/android/frontend/FrontendViewModelTest.kt	Adds tests ensuring the ViewModel reflects changes and respects seeded values
app/src/main/kotlin/io/homeassistant/companion/android/frontend/FrontendScreen.kt	Collects keepScreenOnEnabled and applies it via KeepScreenOnEffect (View.keepScreenOn)