settings: add the replay gain controls

Final slice: the screen that drives the mode and preamp preferences the
playback side already reads. Until now `setReplayGainMode` and
`setReplayGainPreampDb` had no production caller, so Off and the preamp
were unreachable.

- `SettingsViewModel` exposes both preferences and writes them back.
- The Playback section gains a "Replay gain" row opening an Off / Track /
  Album choice, and a "Preamp" row opening a slider across the +-15 dB
  range in 0.5 dB steps.
- `ReplayGainMode` is ordered Off, Track, Album so the choice list reads
  that way; the stored value is the name, so the order is not persisted
  and the default is still Track.
- The labels live in their own file so the signed decibel formatting can
  be tested, as `formatTrackTime` does.

app/src/main/kotlin/com/subtracks/ui/settings/SettingsViewModel.kt

@@ -5,6 +5,7 @@ import androidx.lifecycle.viewModelScope
 import com.subtracks.data.model.Source
 import com.subtracks.data.net.NetworkMode
 import com.subtracks.data.prefs.DEFAULT_SYNC_INTERVAL_MINUTES
+import com.subtracks.data.prefs.ReplayGainMode
 import com.subtracks.data.prefs.StreamQuality
 import com.subtracks.data.prefs.SyncMode
 import com.subtracks.data.prefs.UserPreferences
@@ -65,6 +66,16 @@ class SettingsViewModel(
             .scrobbling()
             .stateIn(viewModelScope, SharingStarted.WhileSubscribed(5_000), true)
 
+    val replayGainMode: StateFlow<ReplayGainMode> =
+        userPreferences
+            .replayGainMode()
+            .stateIn(viewModelScope, SharingStarted.WhileSubscribed(5_000), ReplayGainMode.Track)
+
+    val replayGainPreamp: StateFlow<Float> =
+        userPreferences
+            .replayGainPreampDb()
+            .stateIn(viewModelScope, SharingStarted.WhileSubscribed(5_000), 0f)
+
     val offline: StateFlow<Boolean> = sourceRepository.offline
 
     val verboseLogging: StateFlow<Boolean> =
@@ -115,4 +126,12 @@ class SettingsViewModel(
     fun setScrobbling(enabled: Boolean) {
         viewModelScope.launch { userPreferences.setScrobbling(enabled) }
     }
+
+    fun setReplayGainMode(mode: ReplayGainMode) {
+        viewModelScope.launch { userPreferences.setReplayGainMode(mode) }
+    }
+
+    fun setReplayGainPreamp(value: Float) {
+        viewModelScope.launch { userPreferences.setReplayGainPreampDb(value) }
+    }
 }