fix(agenda): list multi-day events on every day they span

groupAgendaDays keyed each instance by its start day alone, so a
multi-day event surfaced only on its first day and vanished from the
rest of its span in both the Agenda screen and the agenda widget.

Expand each instance across every day from its start (clamped to the
anchor for ongoing events) through its last occupied day, bounded by the
visible window end. An event ending exactly at midnight — including the
exclusive next-midnight all-day events end at — does not reach that
boundary day.

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

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

@@ -0,0 +1,119 @@
+package de.jeanlucmakiola.calendula.ui.agenda
+
+import com.google.common.truth.Truth.assertThat
+import de.jeanlucmakiola.calendula.domain.EventInstance
+import kotlinx.datetime.LocalDate
+import kotlinx.datetime.LocalDateTime
+import kotlinx.datetime.TimeZone
+import kotlinx.datetime.toInstant
+import kotlin.time.Instant
+import org.junit.jupiter.api.Test
+
+class GroupAgendaDaysTest {
+
+    private val zone = TimeZone.UTC
+    private val anchor = LocalDate(2026, 7, 1)
+    // A wide window so clamping to the window end is out of the way unless tested.
+    private val windowEnd = LocalDate(2026, 7, 31)
+
+    private fun at(y: Int, mo: Int, d: Int, h: Int = 0, min: Int = 0): Instant =
+        LocalDateTime(y, mo, d, h, min).toInstant(zone)
+
+    private fun event(
+        id: Long,
+        title: String,
+        start: Instant,
+        end: Instant,
+        isAllDay: Boolean = false,
+    ) = EventInstance(
+        instanceId = id,
+        eventId = id,
+        calendarId = 1,
+        title = title,
+        start = start,
+        end = end,
+        isAllDay = isAllDay,
+        color = 0,
+        location = null,
+    )
+
+    private fun days(instances: List<EventInstance>) =
+        groupAgendaDays(anchor, windowEnd, instances, zone)
+
+    @Test
+    fun `a timed multi-day event lists on every day it spans`() {
+        val e = event(1, "trip", at(2026, 7, 1, 14, 0), at(2026, 7, 3, 10, 0))
+
+        val result = days(listOf(e))
+
+        assertThat(result.map { it.date }).containsExactly(
+            LocalDate(2026, 7, 1),
+            LocalDate(2026, 7, 2),
+            LocalDate(2026, 7, 3),
+        ).inOrder()
+        result.forEach { assertThat(it.events).containsExactly(e) }
+    }
+
+    @Test
+    fun `an all-day multi-day event stops before its exclusive next-midnight end`() {
+        // 1–3 July inclusive: end is the exclusive midnight opening 4 July.
+        val e = event(1, "holiday", at(2026, 7, 1), at(2026, 7, 4), isAllDay = true)
+
+        val result = days(listOf(e))
+
+        assertThat(result.map { it.date }).containsExactly(
+            LocalDate(2026, 7, 1),
+            LocalDate(2026, 7, 2),
+            LocalDate(2026, 7, 3),
+        ).inOrder()
+    }
+
+    @Test
+    fun `a single-day event lists exactly once`() {
+        val e = event(1, "lunch", at(2026, 7, 2, 12, 0), at(2026, 7, 2, 13, 0))
+
+        assertThat(days(listOf(e)).map { it.date })
+            .containsExactly(LocalDate(2026, 7, 2))
+    }
+
+    @Test
+    fun `an event ending exactly at midnight does not reach the next day`() {
+        val e = event(1, "late", at(2026, 7, 1, 22, 0), at(2026, 7, 2, 0, 0))
+
+        assertThat(days(listOf(e)).map { it.date })
+            .containsExactly(LocalDate(2026, 7, 1))
+    }
+
+    @Test
+    fun `an event begun before the anchor is clamped to the anchor day`() {
+        val e = event(1, "ongoing", at(2026, 6, 29, 9, 0), at(2026, 7, 2, 9, 0))
+
+        assertThat(days(listOf(e)).map { it.date }).containsExactly(
+            LocalDate(2026, 7, 1),
+            LocalDate(2026, 7, 2),
+        ).inOrder()
+    }
+
+    @Test
+    fun `an event running past the window is clamped to the last visible day`() {
+        val narrowEnd = LocalDate(2026, 7, 2)
+        val e = event(1, "long", at(2026, 7, 1, 9, 0), at(2026, 7, 5, 9, 0))
+
+        val result = groupAgendaDays(anchor, narrowEnd, listOf(e), zone)
+
+        assertThat(result.map { it.date }).containsExactly(
+            LocalDate(2026, 7, 1),
+            LocalDate(2026, 7, 2),
+        ).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)
+        val timed = event(2, "call", at(2026, 7, 1, 9, 0), at(2026, 7, 1, 10, 0))
+
+        val dayOne = days(listOf(timed, allDay)).first { it.date == anchor }
+
+        assertThat(dayOne.events).containsExactly(allDay, timed).inOrder()
+    }
+}