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/domain/due.go

@@ -0,0 +1,77 @@
+package domain
+
+import (
+	"strings"
+	"time"
+)
+
+// Due dates are stored as "2006-01-02" (all day) or "2006-01-02T15:04" (with time), always in the
+// server's local time zone (TZ, default Europe/Berlin). Both sort correctly as plain strings.
+const (
+	dueDateLayout     = "2006-01-02"
+	dueDateTimeLayout = "2006-01-02T15:04"
+)
+
+// AllDayAlarmHour is when alarms for tasks without a time fire.
+const AllDayAlarmHour = 9
+
+// NormalizeDue brings accepted spellings ("2006-01-02 15:04", seconds, RFC3339) to the stored form.
+// Values it cannot read are kept as they are so no input gets lost.
+func NormalizeDue(raw string) string {
+	raw = strings.TrimSpace(raw)
+	if raw == "" {
+		return ""
+	}
+	if t, hasTime, ok := ParseDue(raw, time.Local); ok {
+		if hasTime {
+			return t.Format(dueDateTimeLayout)
+		}
+		return t.Format(dueDateLayout)
+	}
+	return raw
+}
+
+// ParseDue reads a due date in loc. hasTime is false for all-day dates (then t is midnight).
+func ParseDue(raw string, loc *time.Location) (t time.Time, hasTime, ok bool) {
+	raw = strings.TrimSpace(raw)
+	if t, err := time.ParseInLocation(dueDateLayout, raw, loc); err == nil {
+		return t, false, true
+	}
+	for _, layout := range []string{dueDateTimeLayout, "2006-01-02 15:04", "2006-01-02T15:04:05", "2006-01-02 15:04:05"} {
+		if t, err := time.ParseInLocation(layout, raw, loc); err == nil {
+			return t, true, true
+		}
+	}
+	if t, err := time.Parse(time.RFC3339, raw); err == nil {
+		return t.In(loc), true, true
+	}
+	return time.Time{}, false, false
+}
+
+// DueAlarmAt is the moment the alarm for a due date fires: the due time (09:00 for all-day tasks)
+// minus the user's lead time.
+func DueAlarmAt(raw string, notifyBeforeMinutes int, loc *time.Location) (time.Time, bool) {
+	t, hasTime, ok := ParseDue(raw, loc)
+	if !ok {
+		return time.Time{}, false
+	}
+	if !hasTime {
+		t = time.Date(t.Year(), t.Month(), t.Day(), AllDayAlarmHour, 0, 0, 0, loc)
+	}
+	if notifyBeforeMinutes > 0 {
+		t = t.Add(-time.Duration(notifyBeforeMinutes) * time.Minute)
+	}
+	return t, true
+}
+
+// HumanDue renders a due date for messages: "30.10.2026" or "30.10.2026, 14:30 Uhr".
+func HumanDue(raw string) string {
+	t, hasTime, ok := ParseDue(raw, time.Local)
+	if !ok {
+		return raw
+	}
+	if hasTime {
+		return t.Format("02.01.2006, 15:04") + " Uhr"
+	}
+	return t.Format("02.01.2006")
+}

internal/domain/due_test.go

@@ -0,0 +1,40 @@
+package domain
+
+import (
+	"testing"
+	"time"
+)
+
+func TestNormalizeDue(t *testing.T) {
+	cases := map[string]string{
+		"2026-10-30":          "2026-10-30",
+		"2026-10-30T14:30":    "2026-10-30T14:30",
+		"2026-10-30 14:30":    "2026-10-30T14:30",
+		"2026-10-30T14:30:59": "2026-10-30T14:30",
+		" ":                   "",
+		"nächste Woche":       "nächste Woche",
+	}
+	for in, want := range cases {
+		if got := NormalizeDue(in); got != want {
+			t.Errorf("NormalizeDue(%q) = %q, want %q", in, got, want)
+		}
+	}
+}
+
+func TestDueAlarmAt(t *testing.T) {
+	berlin, err := time.LoadLocation("Europe/Berlin")
+	if err != nil {
+		t.Fatal(err)
+	}
+	at, ok := DueAlarmAt("2026-10-30", 0, berlin)
+	if !ok || !at.Equal(time.Date(2026, 10, 30, 9, 0, 0, 0, berlin)) {
+		t.Errorf("all-day alarm should fire at 09:00 local, got %v", at)
+	}
+	at, _ = DueAlarmAt("2026-10-30T14:30", 15, berlin)
+	if !at.Equal(time.Date(2026, 10, 30, 14, 15, 0, 0, berlin)) {
+		t.Errorf("lead time not applied: %v", at)
+	}
+	if _, ok := DueAlarmAt("irgendwann", 0, berlin); ok {
+		t.Errorf("unreadable due date must not produce an alarm")
+	}
+}