directions: speak each turn on the car's guidance channel

Map apps' own voice prompts can't reach the car over USB, so Gearslip reads
the turn the cluster already gets and says it through the car's directions
sink, which ducks the music. On by default, with a switch in car settings.

gearslip/src/main/java/app/seb3thehacker/gearslip/car/CarSettingsScreen.kt

@@ -53,6 +53,7 @@ fun CarSettingsScreen() {
     val units by CarSettings.units.collectAsState()
     val whatsNew by CarSettings.whatsNewAfterUpdates.collectAsState()
     val pipeAudio by CarSettings.pipeAudio.collectAsState()
+    val speakDirections by CarSettings.speakDirections.collectAsState()
     val autoplay by CarSettings.autoplay.collectAsState()
     val playerTitle by CarSettings.playerTitle.collectAsState()
     val theme by CarSettings.appTheme.collectAsState()
@@ -137,6 +138,18 @@ fun CarSettingsScreen() {
             color = MaterialTheme.colorScheme.onSurfaceVariant,
         )
 
+        ChoiceRow(
+            "Spoken directions",
+            listOf(false to "Off", true to "On"),
+            speakDirections,
+        ) { CarSettings.setSpeakDirections(it) }
+        Text(
+            "Gearslip reads each turn aloud through the car's speakers, and the music dips while it " +
+                "speaks. Turn it off if your map app's own voice already plays in the car.",
+            style = MaterialTheme.typography.bodySmall,
+            color = MaterialTheme.colorScheme.onSurfaceVariant,
+        )
+
         ExperimentalFeatureRow(
             title = "Use the phone's speech recognition",
             description = "Listens through the phone's own speech service instead of Gearslip's. On " +