feat(todo): reminders at a time of their own planned and set in a task's sheet, durations in its date sheet

planReminders plans each reminder at a time of its own of an open task,
whatever its date, under an ID of its own; the notification says
"Reminder" for a task without a time. The task's sheet always has its
reminders' row: those before its time where it has one, those at a time
of their own each with its remove button and "Remind at a time" (a day,
then a time), the permission asked as a first one is chosen. The date
sheet sets a duration once there is a time (none, 15 minutes to 4
hours, as Todoist's), a time cleared taking it with it. Recorded as
ADR 0033, D18 (the developer's, 3 Nov 2026).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

apps/todo/halkyon_todo_flutter/lib/l10n/app_de.arb

@@ -124,6 +124,15 @@
   "remindersChannel": "Erinnerungen an Aufgaben",
   "remindersNotAllowed": "Benachrichtigungen sind für Halkyon Todo aus: Erinnerungen erscheinen, sobald du sie in den Systemeinstellungen erlaubst.",
   "remindersOnThisDevice": "Erinnerungen kommen auf jedem deiner Geräte, das deine Aufgaben öffnet.",
+  "reminderBody": "Erinnerung",
+  "remindersAtTimes": "Zu einer eigenen Uhrzeit",
+  "reminderAddTime": "Zu einer Uhrzeit erinnern",
+  "reminderRemove": "Erinnerung entfernen",
+  "durationTitle": "Dauer",
+  "durationNone": "Keine Dauer",
+  "durationHours": "{hours} Std.",
+  "durationMinutes": "{minutes} Min.",
+  "durationHoursMinutes": "{hours} Std. {minutes} Min.",
   "weekPrevious": "Vorige Woche",
   "weekNext": "Folgende Woche",
   "searchTitle": "Suchen",