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/test/kotlin/com/subtracks/ui/settings/ReplayGainLabelsTest.kt

@@ -0,0 +1,28 @@
+package com.subtracks.ui.settings
+
+import android.content.Context
+import androidx.test.core.app.ApplicationProvider
+import androidx.test.ext.junit.runners.AndroidJUnit4
+import com.subtracks.data.prefs.ReplayGainMode
+import org.junit.Assert.assertEquals
+import org.junit.Test
+import org.junit.runner.RunWith
+
+@RunWith(AndroidJUnit4::class)
+class ReplayGainLabelsTest {
+    private val resources = ApplicationProvider.getApplicationContext<Context>().resources
+
+    @Test
+    fun modeLabelsNameTheGainEachOneUses() {
+        assertEquals("Off", replayGainModeLabel(resources, ReplayGainMode.Off))
+        assertEquals("Track gain", replayGainModeLabel(resources, ReplayGainMode.Track))
+        assertEquals("Album gain", replayGainModeLabel(resources, ReplayGainMode.Album))
+    }
+
+    @Test
+    fun preampLabelsCarryTheSignAndTheUnit() {
+        assertEquals("+3.0 dB", replayGainPreampLabel(resources, 3f))
+        assertEquals("-2.5 dB", replayGainPreampLabel(resources, -2.5f))
+        assertEquals("0.0 dB", replayGainPreampLabel(resources, 0f))
+    }
+}