fix(views): stop single-day all-day events leaking into the next day

All-day events live at UTC midnights with an exclusive end, but coversDay
sliced each day in the device timezone. East of UTC the exclusive end
landed a few hours into the next local day, so a one-day all-day event
(e.g. a birthday) rendered on two days in the day/week/month views — while
the detail and edit screens, which work in UTC, showed it correctly.

Compare all-day coverage in UTC and step the exclusive end back to the
last covered day, mirroring the detail/edit views.

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

app/src/main/java/de/jeanlucmakiola/calendula/ui/week/WeekViewModel.kt

@@ -181,6 +181,18 @@ internal fun weekRange(start: LocalDate, zone: TimeZone): ClosedRange<Instant> {
 
 /** True if this event overlaps the calendar [day] in [zone] (any portion). */
 internal fun EventInstance.coversDay(day: LocalDate, zone: TimeZone): Boolean {
+    if (isAllDay) {
+        // All-day events live at UTC midnights with an exclusive end. Compare
+        // calendar dates in UTC and step the exclusive end back to the last
+        // covered day (mirroring the detail/edit views), so a one-day event
+        // covers exactly its single date. Slicing the day in the device zone
+        // would push the exclusive end a few hours into the next local day
+        // east of UTC, making the event leak onto day + 1.
+        val startDate = start.toLocalDateTime(TimeZone.UTC).date
+        val endExclusive = end.toLocalDateTime(TimeZone.UTC).date
+        val lastDay = maxOf(startDate, endExclusive.minus(1, DateTimeUnit.DAY))
+        return day in startDate..lastDay
+    }
     val dayStart = day.atStartOfDayIn(zone)
     val dayEnd = day.plus(1, DateTimeUnit.DAY).atStartOfDayIn(zone)
     return start < dayEnd && end > dayStart

app/src/test/java/de/jeanlucmakiola/calendula/ui/week/WeekLayoutTest.kt

@@ -62,13 +62,28 @@ class WeekLayoutTest {
         assertThat(ev.coversDay(wed, zone)).isTrue()
         assertThat(ev.coversDay(mon, zone)).isFalse()
 
-        val multiDay = event(at(mon, 22), at(wed, 2), allDay = true)
+        // All-day: UTC midnights, end exclusive. Mon..Tue covers Mon and Tue
+        // but not Wed (the Wed-midnight end is exclusive).
+        val multiDay = event(at(mon, 0), at(wed, 0), allDay = true)
         assertThat(multiDay.coversDay(mon, zone)).isTrue()
         assertThat(multiDay.coversDay(LocalDate(2026, 6, 9), zone)).isTrue()
-        assertThat(multiDay.coversDay(wed, zone)).isTrue()
+        assertThat(multiDay.coversDay(wed, zone)).isFalse()
         assertThat(multiDay.coversDay(LocalDate(2026, 6, 12), zone)).isFalse()
     }
 
+    @Test
+    fun `single-day all-day event does not leak into the next day east of UTC`() {
+        // A birthday on Wed: the provider stores UTC midnights with an exclusive
+        // end (Thu 00:00 UTC). In a zone east of UTC the device-local day must
+        // still resolve to Wed only — never Thu. Regression for the all-day
+        // event appearing on two days in the views.
+        val berlin = TimeZone.of("Europe/Berlin") // UTC+2 in June
+        val ev = event(at(wed, 0), at(wed.plusDays(1), 0), allDay = true)
+        assertThat(ev.coversDay(wed, berlin)).isTrue()
+        assertThat(ev.coversDay(wed.plusDays(1), berlin)).isFalse()
+        assertThat(ev.coversDay(wed.plusDays(-1), berlin)).isFalse()
+    }
+
     @Test
     fun `single timed event gets one lane`() {
         val blocks = layoutDay(listOf(event(at(wed, 9), at(wed, 10, 30))), wed, zone)