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/alarm_test.go

@@ -0,0 +1,29 @@
+package api
+
+import (
+	"testing"
+	"time"
+)
+
+func TestAlarmDue(t *testing.T) {
+	berlin, _ := time.LoadLocation("Europe/Berlin")
+	at := func(h, m int) time.Time { return time.Date(2026, 10, 30, h, m, 0, 0, berlin) }
+	cases := []struct {
+		due    string
+		before int
+		now    time.Time
+		want   bool
+	}{
+		{"2026-10-30", 0, at(8, 59), false},
+		{"2026-10-30", 0, at(9, 0), true},
+		{"2026-10-30T14:30", 0, at(14, 29), false},
+		{"2026-10-30T14:30", 30, at(14, 0), true},
+		{"2026-10-29", 0, at(0, 1), true},
+		{"bald", 0, at(23, 0), false},
+	}
+	for _, c := range cases {
+		if got := alarmDue(c.due, c.before, c.now); got != c.want {
+			t.Errorf("alarmDue(%q, %d, %v) = %v, want %v", c.due, c.before, c.now.Format("15:04"), got, c.want)
+		}
+	}
+}

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
 						}