feat(domain): add core use cases

app/src/main/java/com/radiola/domain/usecase/GetNowPlayingUseCase.kt

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+package com.radiola.domain.usecase
+
+import com.radiola.domain.model.Track
+import com.radiola.domain.repository.NowPlayingRepository
+import kotlinx.coroutines.flow.Flow
+import javax.inject.Inject
+
+class GetNowPlayingUseCase @Inject constructor(
+    private val nowPlayingRepository: NowPlayingRepository
+) {
+    operator fun invoke(stationPrefix: String): Flow<Track?> {
+        return nowPlayingRepository.getNowPlaying(stationPrefix)
+    }
+}

app/src/main/java/com/radiola/domain/usecase/GetStationsUseCase.kt

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+package com.radiola.domain.usecase
+
+import com.radiola.domain.model.Station
+import com.radiola.domain.repository.StationRepository
+import kotlinx.coroutines.flow.Flow
+import javax.inject.Inject
+
+class GetStationsUseCase @Inject constructor(
+    private val stationRepository: StationRepository
+) {
+    operator fun invoke(): Flow<List<Station>> = stationRepository.getStations()
+}

app/src/main/java/com/radiola/domain/usecase/PlayStationUseCase.kt

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+package com.radiola.domain.usecase
+
+import com.radiola.domain.model.Station
+import com.radiola.domain.repository.SettingsRepository
+import javax.inject.Inject
+
+class PlayStationUseCase @Inject constructor(
+    private val settingsRepository: SettingsRepository
+) {
+    suspend operator fun invoke(station: Station) {
+        settingsRepository.setLastStationId(station.id)
+        // Actual playback is delegated to PlayerService
+    }
+}

app/src/main/java/com/radiola/domain/usecase/SearchTrackInServiceUseCase.kt

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+package com.radiola.domain.usecase
+
+import com.radiola.domain.model.DeeplinkService
+import com.radiola.domain.model.Track
+import javax.inject.Inject
+
+class SearchTrackInServiceUseCase @Inject constructor() {
+    operator fun invoke(track: Track, service: DeeplinkService): String {
+        return service.buildSearchUrl(track.artist, track.song)
+    }
+}

app/src/main/java/com/radiola/domain/usecase/ToggleFavoriteUseCase.kt

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+package com.radiola.domain.usecase
+
+import com.radiola.domain.model.Station
+import com.radiola.domain.repository.FavoritesRepository
+import kotlinx.coroutines.flow.first
+import javax.inject.Inject
+
+class ToggleFavoriteUseCase @Inject constructor(
+    private val favoritesRepository: FavoritesRepository
+) {
+    suspend operator fun invoke(station: Station) {
+        val isFav = favoritesRepository.isFavorite(station.id).first()
+        if (isFav) {
+            favoritesRepository.removeFavorite(station.id)
+        } else {
+            favoritesRepository.addFavorite(station)
+        }
+    }
+}