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/api/server.go

@@ -3199,6 +3199,13 @@ func (s *Server) sendCustomWebhookNotification(webhookURL, event, title, notes,
 	return nil
 }
 
+// alarmDue reports whether the alarm for a due date should go out now: from the due time (09:00 for
+// all-day tasks) minus the user's lead time on. Unreadable dates never fire.
+func alarmDue(dueDate string, notifyBeforeMinutes int, now time.Time) bool {
+	at, ok := domain.DueAlarmAt(dueDate, notifyBeforeMinutes, now.Location())
+	return ok && !now.Before(at)
+}
+
 func (s *Server) startAlarmTicker() {
 	ticker := time.NewTicker(60 * time.Second)
 	defer ticker.Stop()
@@ -3209,10 +3216,10 @@ func (s *Server) startAlarmTicker() {
 			continue
 		}
 
-		today := time.Now().Format("2006-01-02")
+		now := time.Now()
 
 		for _, a := range alarms {
-			if a.DueDate > today {
+			if !alarmDue(a.DueDate, a.NotifyBeforeMinutes, now) {
 				continue
 			}
 			if len(a.AlarmChannels) == 0 {
@@ -3220,6 +3227,7 @@ func (s *Server) startAlarmTicker() {
 			}
 
 			sentAny := false
+			due := domain.HumanDue(a.DueDate)
 			desc := a.Notes
 			if desc == "" {
 				desc = "Aufgabe ist jetzt fällig."
@@ -3229,7 +3237,7 @@ func (s *Server) startAlarmTicker() {
 				switch ch {
 				case "discord":
 					if a.DiscordWebhookURL != "" {
-						if err := s.sendDiscordNotification(a.DiscordWebhookURL, a.Title, desc, a.DueDate, a.Priority); err == nil {
+						if err := s.sendDiscordNotification(a.DiscordWebhookURL, a.Title, desc, due, a.Priority); err == nil {
 							sentAny = true
 						}
 					}
@@ -3241,7 +3249,7 @@ func (s *Server) startAlarmTicker() {
 					}
 				case "email":
 					if a.AlarmEmail != "" {
-						emailBody := fmt.Sprintf("Hallo,\n\ndeine Aufgabe in MindSift ist fällig:\n\nAufgabe: %s\nFällig am: %s\nPriorität: %s\nNotizen: %s\n", a.Title, a.DueDate, a.Priority, desc)
+						emailBody := fmt.Sprintf("Hallo,\n\ndeine Aufgabe in MindSift ist fällig:\n\nAufgabe: %s\nFällig am: %s\nPriorität: %s\nNotizen: %s\n", a.Title, due, a.Priority, desc)
 						if err := s.sendEmailNotification(a.AlarmEmail, "⏰ Aufgabe fällig: "+a.Title, emailBody); err == nil {
 							sentAny = true
 						}