perf(todo): ticks, drags and the chosen tasks show at once, written in one go

What a person does to their tasks shows before the device has sealed
and kept it (Flutter's optimistic state): TodoCubit.showing shows a
change on the tasks at once and on every state read while its write
runs; where the write fails, the tasks show as they are again at once
and a snack bar says so (the keys missing, or "That wasn't saved"),
through writeTasks. A task's check, a task dragged to a day, an hour or
a place in its project, Reschedule, and the selection's date, labels,
priority, deadline, move, complete and delete go through it; Undo stays
where it was, offered once the write is kept.

The chosen tasks were written one at a time, each write opening every
task again (SealedRows.all decrypts all of them), so deleting a hundred
opened them a hundred times. TaskRepository now reads once and writes
together: updateAll, completeAll, deleteAll, move, place and restore,
the automatic reminder asked once, and SealedRows.updateAll seals all
of them before writing any (KeysMissing leaves none written) and fires
one change, so the screen reads once after. What a write changes is
worked out by pure functions shared by the repository and the screen
(movedTo, placedAt, completedContent, withSubTasks in task_views.dart).

Whats-New: Ticking, moving and changing tasks, many at once too, shows at once.
Whats-New-De: Abhaken, Verschieben und Ändern von Aufgaben, auch vieler auf einmal, zeigt sich sofort.
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_localizations.dart

@@ -286,6 +286,12 @@ abstract class AppLocalizations {
   /// **'This device can\'t save: it doesn\'t hold your account\'s keys. Sign in with your password or recovery phrase to add them.'**
   String get keysMissing;
 
+  /// A snack bar after a change to tasks, shown at once, could not be kept on the device and was taken back.
+  ///
+  /// In en, this message translates to:
+  /// **'That wasn\'t saved, so your tasks are as they were. Try again.'**
+  String get tasksNotSaved;
+
   /// No description provided for @filtersLabelsTitle.
   ///
   /// In en, this message translates to: