feat(todo): tasks dragged into place or under another, the add button dropped where the task goes (ADR 0033, D16)

- A project's row, held, is dragged before or after another, or a
  little to the right to become its sub-task, or onto a section's
  heading; its sub-tasks go with it, and Order changed offers Undo
  (TaskRepository.place). The list scrolls at its edges.
- The add button is dragged onto a project's list to add the task
  between two rows, onto a day in Upcoming for that day, onto Today
  for today; dropped back on its place, an ×, it gives up.
- Holding a row now drags it; a swipe to the right selects it.

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

@@ -1095,6 +1095,18 @@ abstract class AppLocalizations {
   /// In en, this message translates to:
   /// **'Deadline'**
   String get deadlineTitle;
+
+  /// Snack bar after a task was dragged into another place (ADR 0033, D16).
+  ///
+  /// In en, this message translates to:
+  /// **'Order changed'**
+  String get orderChanged;
+
+  /// The × left in the add button's place while it is dragged (D16).
+  ///
+  /// In en, this message translates to:
+  /// **'Drop here to cancel'**
+  String get addTaskCancel;
 }
 
 class _AppLocalizationsDelegate