fix(agenda): resolve all-day span days in UTC, not the device zone

Review follow-up. spanFirstDay/spanLastDay resolved every event in the
device zone, but all-day events live at UTC midnights with an exclusive
end — east of UTC (e.g. Europe/Berlin) that pushed spanLastDay onto the
next day, so a single-day all-day event reported spansMultipleDays and
leaked onto a second agenda day. Resolve all-day dates in UTC, matching
the Week view and detail card. Adds eastern-zone regression tests that
the prior UTC-only tests could not catch.

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

app/src/main/java/de/jeanlucmakiola/calendula/ui/agenda/AgendaUiState.kt

@@ -9,18 +9,27 @@ import kotlinx.datetime.plus
 import kotlinx.datetime.toLocalDateTime
 import kotlin.time.Duration.Companion.milliseconds
 
-/** The first calendar day this event occupies, in [zone]. */
+/**
+ * The zone the event's calendar dates live in. Timed events are resolved in the
+ * device [zone]; all-day events live at UTC midnights with an exclusive end, so
+ * resolving them anywhere but UTC shifts the boundaries — east of UTC that leaks
+ * a one-day event onto its next day. Matches the Week view and detail card.
+ */
+private fun EventInstance.dateZone(zone: TimeZone): TimeZone =
+    if (isAllDay) TimeZone.UTC else zone
+
+/** The first calendar day this event occupies. */
 fun EventInstance.spanFirstDay(zone: TimeZone): LocalDate =
-    start.toLocalDateTime(zone).date
+    start.toLocalDateTime(dateZone(zone)).date
 
 /**
- * The last calendar day this event actually occupies, in [zone]. An event ending
- * exactly at midnight (all-day events end at the exclusive next-midnight) does
- * not reach into that boundary day, so resolve the instant just before [end].
+ * The last calendar day this event actually occupies. An event ending exactly at
+ * midnight (all-day events end at the exclusive next-midnight) does not reach
+ * into that boundary day, so resolve the instant just before [end].
  */
 fun EventInstance.spanLastDay(zone: TimeZone): LocalDate {
     val lastInstant = if (end > start) end - 1.milliseconds else start
-    return lastInstant.toLocalDateTime(zone).date
+    return lastInstant.toLocalDateTime(dateZone(zone)).date
 }
 
 /** Whether this event occupies more than one calendar day in [zone]. */

app/src/test/java/de/jeanlucmakiola/calendula/ui/agenda/GroupAgendaDaysTest.kt

@@ -107,6 +107,37 @@ class GroupAgendaDaysTest {
         ).inOrder()
     }
 
+    // All-day events are stored at UTC midnights; resolving them in a device
+    // zone east of UTC would leak them onto the following day (see spanLastDay).
+    private val berlin = TimeZone.of("Europe/Berlin")
+
+    private fun utcMidnight(y: Int, mo: Int, d: Int): Instant =
+        LocalDateTime(y, mo, d, 0, 0).toInstant(TimeZone.UTC)
+
+    @Test
+    fun `a single-day all-day event does not leak onto the next day in an eastern zone`() {
+        val e = event(1, "birthday", utcMidnight(2026, 7, 2), utcMidnight(2026, 7, 3), isAllDay = true)
+
+        val result = groupAgendaDays(anchor, windowEnd, listOf(e), berlin)
+
+        assertThat(result.map { it.date }).containsExactly(LocalDate(2026, 7, 2))
+        assertThat(e.spansMultipleDays(berlin)).isFalse()
+    }
+
+    @Test
+    fun `an all-day multi-day event spans its true days in an eastern zone`() {
+        // 2–4 July inclusive: exclusive end is the UTC midnight opening 5 July.
+        val e = event(1, "holiday", utcMidnight(2026, 7, 2), utcMidnight(2026, 7, 5), isAllDay = true)
+
+        val result = groupAgendaDays(anchor, windowEnd, listOf(e), berlin)
+
+        assertThat(result.map { it.date }).containsExactly(
+            LocalDate(2026, 7, 2),
+            LocalDate(2026, 7, 3),
+            LocalDate(2026, 7, 4),
+        ).inOrder()
+    }
+
     @Test
     fun `within a day all-day events sort before timed ones`() {
         val allDay = event(1, "birthday", at(2026, 7, 1), at(2026, 7, 2), isAllDay = true)