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_en.arb

@@ -449,5 +449,13 @@
   "deadlineTitle": "Deadline",
   "@deadlineTitle": {
     "description": "Quick add's deadline chip and menu entry (D15)."
+  },
+  "orderChanged": "Order changed",
+  "@orderChanged": {
+    "description": "Snack bar after a task was dragged into another place (ADR 0033, D16)."
+  },
+  "addTaskCancel": "Drop here to cancel",
+  "@addTaskCancel": {
+    "description": "The × left in the add button's place while it is dragged (D16)."
   }
 }