feat(todo): Upcoming's tasks dragged onto another day (ADR 0033, D16)

A row of Upcoming, overdue ones too, held and dropped on another day
moves there, its time, duration and recurrence kept; Moved to … offers
Undo. The tint over a day a drag hovers is laid beside the rows, not
around them, which paint on the Material beneath.

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

@@ -167,5 +167,6 @@
   "quickAddTokenHint": "Tippe ein hervorgehobenes Wort an, um es als Text zu behalten.",
   "deadlineTitle": "Frist",
   "orderChanged": "Reihenfolge geändert",
-  "addTaskCancel": "Hier ablegen zum Abbrechen"
+  "addTaskCancel": "Hier ablegen zum Abbrechen",
+  "taskMovedTo": "Verschoben auf {day}"
 }