refactor(settings): remove the unused default timer duration

Settings > Timers offered a "Default duration", but nothing that creates
a timer ever read it: the keypad starts empty. The row, its preference,
ClockDefaults.timerDuration and the backup field are gone, and the
screen's hint and the hub's subtitle now only speak of the sound.

Backups no longer write default_timer_duration_millis. Older files that
carry it still import, since the codec ignores unknown keys.

Closes #5

app/src/test/java/de/jeanlucmakiola/clockula/data/backup/BackupCodecTest.kt

@@ -58,7 +58,6 @@ class BackupCodecTest {
                 alarmRingtoneUri = "content://media/1",
                 timerRingtoneUri = "content://media/2",
                 dismissChallenge = DismissChallenge.HOLD,
-                timerDuration = 10.minutes,
                 homeZoneId = "America/New_York",
             ),
             timerPresets = listOf(1.minutes, 2.minutes, 10.minutes),
@@ -113,6 +112,32 @@ class BackupCodecTest {
         assertThat(decoded).isInstanceOf(BackupResult.Success::class.java)
     }
 
+    @Test
+    fun `an older file with the retired default timer duration still imports`() {
+        val json = """
+            {
+              "format": "clockula-backup",
+              "version": 1,
+              "exportedAt": 0,
+              "alarms": [],
+              "timers": [],
+              "worldClocks": [],
+              "settings": { "default_snooze_minutes": 7, "default_timer_duration_millis": 600000 }
+            }
+        """.trimIndent()
+
+        val decoded = BackupCodec.decode(json)
+
+        assertThat(decoded).isInstanceOf(BackupResult.Success::class.java)
+        val defaults = (decoded as BackupResult.Success).content.settings.defaults
+        assertThat(defaults).isEqualTo(ClockDefaults(snoozeMinutes = 7))
+    }
+
+    @Test
+    fun `the retired default timer duration is no longer written`() {
+        assertThat(BackupCodec.encode(sampleContent())).doesNotContain("default_timer_duration_millis")
+    }
+
     @Test
     fun `wrong format rejects the whole file`() {
         val decoded = BackupCodec.decode(minimalDocument(format = "something-else"))

app/src/test/java/de/jeanlucmakiola/clockula/data/prefs/ClockPrefsTest.kt

@@ -20,7 +20,6 @@ import kotlinx.coroutines.test.runTest
 import org.junit.jupiter.api.Test
 import org.junit.jupiter.api.io.TempDir
 import java.nio.file.Path
-import kotlin.time.Duration.Companion.seconds
 
 /**
  * The clock defaults every alarm and timer inherits. A value the user could
@@ -53,7 +52,6 @@ class ClockPrefsTest {
         prefs.setAlarmRingtoneUri("content://a")
         prefs.setTimerRingtoneUri("content://t")
         prefs.setDismissChallenge(DismissChallenge.HOLD)
-        prefs.setTimerDuration(90.seconds)
         prefs.setHomeZoneId("Europe/Berlin")
 
         assertThat(prefs.defaults.first()).isEqualTo(
@@ -65,7 +63,6 @@ class ClockPrefsTest {
                 alarmRingtoneUri = "content://a",
                 timerRingtoneUri = "content://t",
                 dismissChallenge = DismissChallenge.HOLD,
-                timerDuration = 90.seconds,
                 homeZoneId = "Europe/Berlin",
             ),
         )

app/src/test/java/de/jeanlucmakiola/clockula/ui/settings/SettingsViewModelTest.kt

@@ -21,7 +21,6 @@ import org.junit.jupiter.api.Test
 import org.junit.jupiter.api.extension.RegisterExtension
 import org.junit.jupiter.api.io.TempDir
 import java.nio.file.Path
-import kotlin.time.Duration.Companion.minutes
 
 /**
  * The real [SettingsViewModel] over the real [de.jeanlucmakiola.clockula.data.prefs.SettingsPrefs]
@@ -120,7 +119,6 @@ class SettingsViewModelTest {
         viewModel.onSnoozeLimitChanged(2)
         viewModel.onVolumeRampChanged(30)
         viewModel.onDismissChallengeChanged(DismissChallenge.MATH)
-        viewModel.onTimerDurationChanged(20.minutes)
         advanceUntilIdle()
 
         val defaults = (viewModel.state.value as SettingsUiState.Loaded).defaults
@@ -129,7 +127,6 @@ class SettingsViewModelTest {
         assertThat(defaults.snoozeLimit).isEqualTo(2)
         assertThat(defaults.volumeRampSeconds).isEqualTo(30)
         assertThat(defaults.dismissChallenge).isEqualTo(DismissChallenge.MATH)
-        assertThat(defaults.timerDuration).isEqualTo(20.minutes)
     }
 
     @Test