fix(settings): make the reminder picker's empty state safe and explicit

Three defects in the v2.13.0 multi-select reminder picker:

- Unchecking the last time on a per-calendar picker silently persisted
  an explicit no-reminder override; an accidental toggle-undo wiped the
  calendar's default. Empty-by-unchecking now reverts to 'Use default'
  (Inherit); deliberate no-reminder is its own exclusive 'None' row
  (reusing reminder_none) on both pickers, so the empty state is
  visible and reachable instead of implicit.

- A custom (non-preset) lead time's row vanished the moment it was
  unchecked, stranding the hand-entered value. Custom values seen this
  session keep their row (unchecked) until the picker closes.

- The optimistic selection seeded once from a possibly-not-yet-loaded
  settings state (initialValue emptyList behind a CalendarProvider-
  gated combine), so a quick first toggle after process-death restore
  overwrote the stored default. The local state now re-syncs from the
  incoming selection until the user first interacts.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/src/test/java/de/jeanlucmakiola/calendula/ui/common/ReminderOverrideForMinutesTest.kt

@@ -0,0 +1,35 @@
+package de.jeanlucmakiola.calendula.ui.common
+
+import com.google.common.truth.Truth.assertThat
+import de.jeanlucmakiola.calendula.data.prefs.CalendarReminderOverride
+import org.junit.jupiter.api.Test
+
+/**
+ * The pure emit rule behind [ReminderDefaultPicker]: a non-empty selection is
+ * [CalendarReminderOverride.Minutes] (normalised), while clearing the last time
+ * reverts to [CalendarReminderOverride.Inherit] on a per-calendar picker (so an
+ * accidental toggle-undo can't silently wipe the calendar's default) and to
+ * explicit [CalendarReminderOverride.None] on the global default.
+ */
+class ReminderOverrideForMinutesTest {
+
+    @Test
+    fun `empty selection on a per-calendar picker reverts to inherit`() {
+        assertThat(reminderOverrideForMinutes(emptyList(), allowInherit = true))
+            .isEqualTo(CalendarReminderOverride.Inherit)
+    }
+
+    @Test
+    fun `empty selection on the global default is explicit none`() {
+        assertThat(reminderOverrideForMinutes(emptyList(), allowInherit = false))
+            .isEqualTo(CalendarReminderOverride.None)
+    }
+
+    @Test
+    fun `a non-empty selection is normalised minutes regardless of scope`() {
+        assertThat(reminderOverrideForMinutes(listOf(30, 10, 10, 60), allowInherit = true))
+            .isEqualTo(CalendarReminderOverride.Minutes(listOf(10, 30, 60)))
+        assertThat(reminderOverrideForMinutes(listOf(30, 10, 10, 60), allowInherit = false))
+            .isEqualTo(CalendarReminderOverride.Minutes(listOf(10, 30, 60)))
+    }
+}