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/feature/settings/settings_payload_test.dart

@@ -68,4 +68,48 @@ void main() {
     );
     expect(decodeSettings(payload({'schemaVersion': 2})), isNull);
   });
+
+  test("D4's settings of tasks and reminders read back as they were set", () {
+    const settings = TodoSettings(
+      tasks: TaskSettings(weekStart: DateTime.sunday, resetSubTasks: false),
+      reminders: ReminderSettings(automatic: 30, snooze: 10),
+    );
+    expect(decodeSettings(encodeSettings(settings)), settings);
+    expect(written(settings), {
+      'schemaVersion': 1,
+      'tasks': {'weekStart': 7, 'resetSubTasks': false},
+      'reminders': {'automatic': 30, 'snooze': 10},
+    });
+    expect(
+      decodeSettings(
+        encodeSettings(
+          const TodoSettings(reminders: ReminderSettings(automatic: 0)),
+        ),
+      ),
+      const TodoSettings(reminders: ReminderSettings(automatic: 0)),
+      reason: 'at the time itself is a reminder too',
+    );
+  });
+
+  test('at first: the week from the language, sub-tasks reset, no automatic '
+      'reminder, snoozed for 15 minutes', () {
+    const settings = TodoSettings();
+    expect(settings.tasks.weekStart, isNull);
+    expect(settings.tasks.resetSubTasks, isTrue);
+    expect(settings.reminders.automatic, isNull);
+    expect(settings.reminders.snooze, 15);
+  });
+
+  test('values out of range read as at first', () {
+    expect(
+      decodeSettings(
+        payload({
+          'schemaVersion': 1,
+          'tasks': {'weekStart': 9, 'resetSubTasks': 'yes'},
+          'reminders': {'automatic': -5, 'snooze': 0},
+        }),
+      ),
+      const TodoSettings(),
+    );
+  });
 }