feat(todo): Settings' Tasks and Reminders, with Snooze on a reminder (ADR 0033, D4)

Settings gains Todo's own groups, kept in the person's sealed settings
row on every device. Tasks: the weekday weeks start on (the language's
unless set), which the week strip, the date and repeat pickers and the
filters follow; and whether a recurring task done opens its sub-tasks
again (on at first). Reminders: an automatic reminder (none at first)
that TaskRepository writes into a task gaining a time without reminders
of its own, and how long a reminder is snoozed for (15 minutes at first).

Each reminder's notification has a Snooze action on Android and Linux:
the reminder comes back that much later under an ID of its own,
scheduled from what the notification carries, so on Android with the
app closed too (the plugin's ActionBroadcastReceiver declared); the plan
of the person's reminders leaves snoozed ones alone. iOS, macOS and
Windows get it later.

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/test/support/memory_reminder_gateway.dart

@@ -9,6 +9,9 @@ class MemoryReminderGateway() implements ReminderGateway {
   /// How often the scheduled ones were replaced.
   int replaced = 0;
 
+  /// How the scheduled ones are snoozed.
+  ({int minutes, String label})? snooze;
+
   /// What the person answers when asked.
   bool allowed = true;
 
@@ -19,8 +22,10 @@ class MemoryReminderGateway() implements ReminderGateway {
   Future<void> replace(
     List<ReminderNotice> notices, {
     required String channel,
+    ({int minutes, String label})? snooze,
   }) async {
     scheduled = [...notices];
+    this.snooze = snooze;
     replaced++;
   }