feat(tasks): due dates with optional time; alarms at the right moment

Due dates can carry a time (YYYY-MM-DDTHH:MM). Alarms fire at that time,
or 09:00 for all-day tasks, minus the user's lead time, in TZ (default
Europe/Berlin, tzdata embedded). A new due date or reopening a task
re-arms the alarm. Recurrence keeps the time; Markdown export/import
carry it. The UI shows relative labels ("Heute, 14:30"), marks overdue
tasks to the minute and groups by local day instead of the UTC date.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/storage/recurrence.go

@@ -18,12 +18,14 @@ func validRecurrence(r string) bool {
 	return false
 }
 
-// NextDueDate moves a due date (YYYY-MM-DD, or today when empty) one interval forward. Months and years
-// keep the day where possible and fall back to the month's last day (31 Jan -> 28/29 Feb).
+// NextDueDate moves a due date (YYYY-MM-DD or YYYY-MM-DDTHH:MM, today when empty) one interval forward
+// and keeps the time. Months and years keep the day where possible and fall back to the month's last
+// day (31 Jan -> 28/29 Feb).
 func NextDueDate(due, rule string, today time.Time) string {
-	base, err := time.Parse("2006-01-02", due)
-	if err != nil {
-		base = time.Date(today.Year(), today.Month(), today.Day(), 0, 0, 0, 0, time.UTC)
+	parsed, hasTime, ok := domain.ParseDue(due, time.UTC)
+	base := time.Date(today.Year(), today.Month(), today.Day(), 0, 0, 0, 0, time.UTC)
+	if ok {
+		base = time.Date(parsed.Year(), parsed.Month(), parsed.Day(), 0, 0, 0, 0, time.UTC)
 	}
 	switch rule {
 	case "daily":
@@ -35,6 +37,9 @@ func NextDueDate(due, rule string, today time.Time) string {
 	case "yearly":
 		base = addMonthsClamped(base, 12)
 	}
+	if hasTime {
+		return base.Format("2006-01-02") + parsed.Format("T15:04")
+	}
 	return base.Format("2006-01-02")
 }