fix(web): account for Google Calendar's exclusive all-day end-date

A single-day all-day event's End.Date is one calendar day past its own
day (Google Calendar convention, already documented by
TestParseEventTime_AllDayEvent). isMultiDayEvent/multiDayVariant didn't
adjust for this, so every all-day event -- including single-day ones --
was misclassified as multi-day and duplicated across Today and
Tomorrow.

2 files changed, 65 insertions, 2 deletions

diff --git a/internal/handlers/timeline.go b/internal/handlers/timeline.go
index e8eadde..9656d4c 100644
--- a/internal/handlers/timeline.go
+++ b/internal/handlers/timeline.go
@@ -24,6 +24,21 @@ func dateOnly(t time.Time) time.Time {
 	return time.Date(t.Year(), t.Month(), t.Day(), 0, 0, 0, 0, t.Location())
 }
 
+// effectiveEndDay returns the last calendar day the item's occurrence
+// actually covers. Google Calendar's all-day End.Date is EXCLUSIVE -- a
+// single-day all-day event on July 13 has End = July 14 at midnight (see
+// TestParseEventTime_AllDayEvent in internal/api/google_calendar_test.go)
+// -- so for an all-day item this subtracts one day to get the real last
+// day. Timed events' End is already the real end instant, so no
+// adjustment is needed there.
+func effectiveEndDay(item models.TimelineItem, tz *time.Location) time.Time {
+	endDay := dateOnly(item.EndTime.In(tz))
+	if item.IsAllDay {
+		return endDay.AddDate(0, 0, -1)
+	}
+	return endDay
+}
+
 // isMultiDayEvent reports whether item is a calendar event whose Start and
 // End fall on different calendar days (in the display timezone). Only
 // events are ever considered multi-day -- tasks, cards, meals, and Google
@@ -33,7 +48,9 @@ func isMultiDayEvent(item models.TimelineItem) bool {
 		return false
 	}
 	tz := config.GetDisplayTimezone()
-	return !dateOnly(item.EndTime.In(tz)).Equal(dateOnly(item.Time.In(tz)))
+	startDay := dateOnly(item.Time.In(tz))
+	endDay := effectiveEndDay(item, tz)
+	return endDay.After(startDay)
 }
 
 // multiDayVariant returns how a multi-day item should render on renderDay:
@@ -44,7 +61,7 @@ func isMultiDayEvent(item models.TimelineItem) bool {
 func multiDayVariant(item models.TimelineItem, renderDay time.Time) string {
 	tz := config.GetDisplayTimezone()
 	startDay := dateOnly(item.Time.In(tz))
-	endDay := dateOnly(item.EndTime.In(tz))
+	endDay := effectiveEndDay(item, tz)
 	day := dateOnly(renderDay.In(tz))
 
 	switch {
diff --git a/internal/handlers/timeline_multiday_test.go b/internal/handlers/timeline_multiday_test.go
index 3dc2005..7af2cbe 100644
--- a/internal/handlers/timeline_multiday_test.go
+++ b/internal/handlers/timeline_multiday_test.go
@@ -81,3 +81,49 @@ func TestMultiDayVariant_DayOutsideSpan_ReturnsEmpty(t *testing.T) {
 		t.Errorf("variant = %q, want empty", got)
 	}
 }
+
+func TestIsMultiDayEvent_SingleDayAllDayEvent_False(t *testing.T) {
+	// Exclusive end-date convention: a single-day all-day event on July 13
+	// has Start=Jul13 00:00, End=Jul14 00:00 (one day past its own day).
+	start, _ := time.Parse(time.RFC3339, "2026-07-13T00:00:00Z")
+	end, _ := time.Parse(time.RFC3339, "2026-07-14T00:00:00Z")
+	item := models.TimelineItem{
+		ID: "allday-1", Type: models.TimelineItemTypeEvent,
+		Time: start, EndTime: &end, IsAllDay: true,
+	}
+	if isMultiDayEvent(item) {
+		t.Error("expected a single-day all-day event (exclusive end-date) to not be multi-day")
+	}
+}
+
+func TestIsMultiDayEvent_MultiDayAllDayEvent_True(t *testing.T) {
+	// A 3-day all-day event (Jul 13, 14, 15) has End=Jul16 00:00 (exclusive).
+	start, _ := time.Parse(time.RFC3339, "2026-07-13T00:00:00Z")
+	end, _ := time.Parse(time.RFC3339, "2026-07-16T00:00:00Z")
+	item := models.TimelineItem{
+		ID: "allday-2", Type: models.TimelineItemTypeEvent,
+		Time: start, EndTime: &end, IsAllDay: true,
+	}
+	if !isMultiDayEvent(item) {
+		t.Error("expected a 3-day all-day event to be multi-day")
+	}
+}
+
+func TestMultiDayVariant_MultiDayAllDayEvent_LastRealDayIsEnds(t *testing.T) {
+	// The 3-day all-day event above (Jul 13-15 inclusive, End=Jul16 exclusive)
+	// must report "ends" on Jul 15 (its real last day), not Jul 16.
+	start, _ := time.Parse(time.RFC3339, "2026-07-13T00:00:00Z")
+	end, _ := time.Parse(time.RFC3339, "2026-07-16T00:00:00Z")
+	item := models.TimelineItem{
+		ID: "allday-2", Type: models.TimelineItemTypeEvent,
+		Time: start, EndTime: &end, IsAllDay: true,
+	}
+	renderDay, _ := time.Parse(time.RFC3339, "2026-07-15T00:00:00Z")
+	if got := multiDayVariant(item, renderDay); got != "ends" {
+		t.Errorf("variant on last real day = %q, want %q", got, "ends")
+	}
+	exclusiveEndDay, _ := time.Parse(time.RFC3339, "2026-07-16T00:00:00Z")
+	if got := multiDayVariant(item, exclusiveEndDay); got != "" {
+		t.Errorf("variant on the exclusive (non-real) end day = %q, want empty", got)
+	}
+}