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_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")
+	}
+}