fix(agenda): share the day-aware time label with the widget

Code-review follow-up.

- The multi-day expansion in groupAgendaDays is shared with the agenda
  widget, but only the screen's summary was made day-aware — so the
  widget rendered the raw "start – end" on every spanned day, the very
  bug the screen fix cured. Hoist a pure agendaTimeLabel(event, day,
  zone) into the shared agenda layer and resolve strings from it in both
  the screen and the widget, so they label identically. (findings 1, 2)

- groupAgendaDays could silently drop an instance whose clamped span was
  empty (firstDay > lastDay); floor lastDay at firstDay so a returned
  instance always surfaces on at least its first visible day. (finding 3)

- agendaTimeLabel resolves the span days once instead of the summary
  recomputing them 2–3× per row. (finding 5)

Finding 4 (within-day sort) needs no change: sorting by absolute start
already places a still-running multi-day event at the top of each day it
continues into, which is chronologically correct (it is ongoing from
that day's midnight), and at its real start time on its first day.

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

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

@@ -0,0 +1,76 @@
+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 AgendaTimeLabelTest {
+
+    private val zone = TimeZone.UTC
+
+    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(start: Instant, end: Instant, isAllDay: Boolean = false) = EventInstance(
+        instanceId = 1,
+        eventId = 1,
+        calendarId = 1,
+        title = "e",
+        start = start,
+        end = end,
+        isAllDay = isAllDay,
+        color = 0,
+        location = null,
+    )
+
+    private fun labelOn(y: Int, mo: Int, d: Int, event: EventInstance) =
+        agendaTimeLabel(event, LocalDate(y, mo, d), zone)
+
+    @Test
+    fun `a single-day timed event is a start-end range`() {
+        val e = event(at(2026, 7, 2, 12, 0), at(2026, 7, 2, 13, 0))
+
+        assertThat(labelOn(2026, 7, 2, e))
+            .isEqualTo(AgendaTimeLabel.Range(e.start, e.end))
+    }
+
+    @Test
+    fun `a single-day all-day event is all-day`() {
+        val e = event(at(2026, 7, 2), at(2026, 7, 3), isAllDay = true)
+
+        assertThat(labelOn(2026, 7, 2, e)).isEqualTo(AgendaTimeLabel.AllDay)
+    }
+
+    @Test
+    fun `a multi-day timed event names the start, middle, and end days`() {
+        val e = event(at(2026, 7, 1, 14, 0), at(2026, 7, 4, 10, 0))
+
+        assertThat(labelOn(2026, 7, 1, e)).isEqualTo(AgendaTimeLabel.Starts(e.start))
+        assertThat(labelOn(2026, 7, 2, e)).isEqualTo(AgendaTimeLabel.AllDay)
+        assertThat(labelOn(2026, 7, 3, e)).isEqualTo(AgendaTimeLabel.AllDay)
+        assertThat(labelOn(2026, 7, 4, e)).isEqualTo(AgendaTimeLabel.Ends(e.end))
+    }
+
+    @Test
+    fun `a multi-day all-day event is all-day on every day`() {
+        val e = event(at(2026, 7, 2), at(2026, 7, 5), isAllDay = true)
+
+        assertThat(labelOn(2026, 7, 2, e)).isEqualTo(AgendaTimeLabel.AllDay)
+        assertThat(labelOn(2026, 7, 3, e)).isEqualTo(AgendaTimeLabel.AllDay)
+        assertThat(labelOn(2026, 7, 4, e)).isEqualTo(AgendaTimeLabel.AllDay)
+    }
+
+    @Test
+    fun `an event begun before the shown day is not labelled as starting`() {
+        // Runs 29 Jun 09:00 → 2 Jul 09:00; on 1 Jul it is mid-span, on 2 Jul it ends.
+        val e = event(at(2026, 6, 29, 9, 0), at(2026, 7, 2, 9, 0))
+
+        assertThat(labelOn(2026, 7, 1, e)).isEqualTo(AgendaTimeLabel.AllDay)
+        assertThat(labelOn(2026, 7, 2, e)).isEqualTo(AgendaTimeLabel.Ends(e.end))
+    }
+}