feat(settings): add 12/24-hour time format toggle

Add a TimeFormatPref (Auto/12h/24h) preference. Auto follows the device's
24-hour system setting; the others force a clock app-wide. The resolved
convention is provided once at the app root via LocalUse24HourFormat, so
every in-app time label reads it without per-screen plumbing.

A shared, pure TimeFormat helper (formatTimeOfDay / formatMinuteOfDay /
formatHourLabel / timeOfDayFormatter) is now the single source for all
time-of-day rendering. Routing every site through it also fixes a
pre-existing inconsistency: the week/day timeline gutters and the agenda
screen + widget hard-coded 24h, while event detail/edit/search/reminders
followed the locale — so a 12h-locale user previously saw mixed formats.

Covered: week & day timelines (gutter + event blocks), agenda screen and
widget, event detail/edit, search, and reminder notifications.

Closes #6 (Codeberg)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/src/test/java/de/jeanlucmakiola/calendula/data/reminders/ReminderTimeTextTest.kt

@@ -26,10 +26,24 @@ class ReminderTimeTextTest {
             isAllDay = false,
             zone = berlin,
             locale = Locale.GERMANY,
+            is24Hour = true,
         )
         assertThat(text).isEqualTo("09:30 – 10:00")
     }
 
+    @Test
+    fun `12-hour preference renders an am-pm time range`() {
+        val text = reminderTimeText(
+            beginMillis = millisAt(LocalDateTime.of(2026, 6, 11, 14, 0), berlin),
+            endMillis = millisAt(LocalDateTime.of(2026, 6, 11, 15, 0), berlin),
+            isAllDay = false,
+            zone = berlin,
+            locale = Locale.US,
+            is24Hour = false,
+        )
+        assertThat(text).isEqualTo("2:00 PM – 3:00 PM")
+    }
+
     @Test
     fun `timed event crossing midnight includes both dates`() {
         val text = reminderTimeText(
@@ -38,6 +52,7 @@ class ReminderTimeTextTest {
             isAllDay = false,
             zone = berlin,
             locale = Locale.GERMANY,
+            is24Hour = true,
         )
         assertThat(text).contains("11.06.2026")
         assertThat(text).contains("12.06.2026")
@@ -54,6 +69,7 @@ class ReminderTimeTextTest {
             // 02:00 in Berlin — naive local reading would shift the day.
             zone = berlin,
             locale = Locale.GERMANY,
+            is24Hour = true,
         )
         assertThat(text).isEqualTo("11.06.2026")
     }
@@ -66,6 +82,7 @@ class ReminderTimeTextTest {
             isAllDay = true,
             zone = berlin,
             locale = Locale.GERMANY,
+            is24Hour = true,
         )
         assertThat(text).isEqualTo("11.06.2026 – 12.06.2026")
     }
@@ -79,6 +96,7 @@ class ReminderTimeTextTest {
             isAllDay = true,
             zone = berlin,
             locale = Locale.GERMANY,
+            is24Hour = true,
         )
         assertThat(text).isEqualTo("11.06.2026")
     }

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

@@ -0,0 +1,37 @@
+package de.jeanlucmakiola.calendula.ui.common
+
+import com.google.common.truth.Truth.assertThat
+import org.junit.jupiter.api.Test
+import java.util.Locale
+
+class TimeFormatTest {
+
+    @Test
+    fun `24-hour format is zero-padded HH mm`() {
+        assertThat(formatTimeOfDay(9, 5, is24Hour = true, Locale.US)).isEqualTo("09:05")
+        assertThat(formatTimeOfDay(14, 0, is24Hour = true, Locale.US)).isEqualTo("14:00")
+        assertThat(formatTimeOfDay(0, 0, is24Hour = true, Locale.US)).isEqualTo("00:00")
+    }
+
+    @Test
+    fun `12-hour format adds a localised am-pm marker`() {
+        assertThat(formatTimeOfDay(14, 0, is24Hour = false, Locale.US)).isEqualTo("2:00 PM")
+        assertThat(formatTimeOfDay(0, 30, is24Hour = false, Locale.US)).isEqualTo("12:30 AM")
+        assertThat(formatTimeOfDay(12, 0, is24Hour = false, Locale.US)).isEqualTo("12:00 PM")
+    }
+
+    @Test
+    fun `minute-of-day end-of-day renders 24 00 in 24h and midnight in 12h`() {
+        assertThat(formatMinuteOfDay(1_440, is24Hour = true, Locale.US)).isEqualTo("24:00")
+        assertThat(formatMinuteOfDay(1_440, is24Hour = false, Locale.US)).isEqualTo("12:00 AM")
+        assertThat(formatMinuteOfDay(13 * 60 + 15, is24Hour = true, Locale.US)).isEqualTo("13:15")
+    }
+
+    @Test
+    fun `hour label is zero-padded in 24h and compact am-pm in 12h`() {
+        assertThat(formatHourLabel(13, is24Hour = true, Locale.US)).isEqualTo("13")
+        assertThat(formatHourLabel(13, is24Hour = false, Locale.US)).isEqualTo("1 PM")
+        assertThat(formatHourLabel(1, is24Hour = false, Locale.US)).isEqualTo("1 AM")
+        assertThat(formatHourLabel(0, is24Hour = false, Locale.US)).isEqualTo("12 AM")
+    }
+}