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_de.arb

@@ -165,5 +165,7 @@
   "tasksMenu": "Mehr",
   "quickAddMore": "Mehr",
   "quickAddTokenHint": "Tippe ein hervorgehobenes Wort an, um es als Text zu behalten.",
-  "deadlineTitle": "Frist"
+  "deadlineTitle": "Frist",
+  "orderChanged": "Reihenfolge geändert",
+  "addTaskCancel": "Hier ablegen zum Abbrechen"
 }