fix(agenda): stop yesterday's all-day event surfacing under today (#65)

The agenda window starts at the anchor's local midnight, which east of UTC
is the previous day's 22:00 UTC. All-day events live at UTC midnights with an
exclusive next-midnight end, so yesterday's all-day event (a birthday, say)
still overlaps today's window start and is returned by the provider.
groupAgendaDays then clamped its first day up to the anchor and, via a trailing
coerceAtLeast(firstDay) on the last day, pulled it onto the anchor's "today"
section — the multi-day fix only stopped the forward leak on interior days.

Drop instances whose true last day (resolved in UTC for all-day events) falls
before the anchor, or whose first day falls past the window end: they occupy no
visible day and must not be clamped onto an edge. Adds eastern-zone regression
tests for both edges.

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

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

@@ -138,6 +138,31 @@ class GroupAgendaDaysTest {
         ).inOrder()
     }
 
+    @Test
+    fun `yesterday's all-day event does not surface under the anchor day in an eastern zone`() {
+        // The agenda window starts at the anchor's local midnight, which in Berlin is
+        // 22:00 UTC the day before — so an all-day event that ended yesterday (whose
+        // exclusive UTC-midnight end sits at that instant) is still returned by the
+        // provider. It must not be clamped onto the anchor's "today" section (#65).
+        val jul19 = LocalDate(2026, 7, 19)
+        val e = event(1, "birthday", utcMidnight(2026, 7, 18), utcMidnight(2026, 7, 19), isAllDay = true)
+
+        val result = groupAgendaDays(jul19, LocalDate(2026, 8, 18), listOf(e), berlin)
+
+        assertThat(result).isEmpty()
+    }
+
+    @Test
+    fun `an all-day event that falls entirely past the window end is dropped`() {
+        // The same guard, at the far edge: an instance whose true days are all beyond
+        // windowEnd occupies no visible day and must not be clamped onto the last one.
+        val e = event(1, "birthday", utcMidnight(2026, 7, 3), utcMidnight(2026, 7, 4), isAllDay = true)
+
+        val result = groupAgendaDays(anchor, LocalDate(2026, 7, 2), listOf(e), berlin)
+
+        assertThat(result).isEmpty()
+    }
+
     @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)