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)
+		}
+	}
+}