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>

frontend/src/lib/due.ts

@@ -0,0 +1,50 @@
+// Due dates come as "YYYY-MM-DD" (all day) or "YYYY-MM-DDTHH:MM" (with time), in the server's time zone.
+// Both sort correctly as plain strings.
+
+const pad = (n: number) => String(n).padStart(2, '0');
+
+/** Local calendar day as YYYY-MM-DD (toISOString would give the UTC day). */
+export const localDay = (d: Date = new Date()): string => `${d.getFullYear()}-${pad(d.getMonth() + 1)}-${pad(d.getDate())}`;
+
+export const addDays = (days: number, from: Date = new Date()): string => {
+  const d = new Date(from);
+  d.setDate(d.getDate() + days);
+  return localDay(d);
+};
+
+export const splitDue = (due?: string): { date: string; time: string } => {
+  if (!due) return { date: '', time: '' };
+  const [date, time = ''] = due.split('T');
+  return { date, time: time.slice(0, 5) };
+};
+
+export const joinDue = (date: string, time: string): string => (date ? (time ? `${date}T${time}` : date) : '');
+
+/** A task with a time is overdue once that minute has passed, an all-day task from the next day on. */
+export const isOverdue = (due: string | undefined, now: Date = new Date()): boolean => {
+  if (!due) return false;
+  const { date, time } = splitDue(due);
+  const today = localDay(now);
+  if (date !== today) return date < today;
+  return !!time && time < `${pad(now.getHours())}:${pad(now.getMinutes())}`;
+};
+
+export const isDueToday = (due: string | undefined, now: Date = new Date()): boolean =>
+  !!due && splitDue(due).date === localDay(now);
+
+const WEEKDAYS = ['So.', 'Mo.', 'Di.', 'Mi.', 'Do.', 'Fr.', 'Sa.'];
+
+/** Short label: "Heute, 14:30", "Morgen", "Gestern", "Fr., 30.10." or "30.10.2027" in another year. */
+export const formatDue = (due: string | undefined, now: Date = new Date()): string => {
+  if (!due) return '';
+  const { date, time } = splitDue(due);
+  const [y, m, d] = date.split('-').map(Number);
+  if (!y || !m || !d) return due;
+  let label: string;
+  if (date === localDay(now)) label = 'Heute';
+  else if (date === addDays(1, now)) label = 'Morgen';
+  else if (date === addDays(-1, now)) label = 'Gestern';
+  else if (y === now.getFullYear()) label = `${WEEKDAYS[new Date(y, m - 1, d).getDay()]}, ${pad(d)}.${pad(m)}.`;
+  else label = `${pad(d)}.${pad(m)}.${y}`;
+  return time ? `${label}, ${time}` : label;
+};