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_en.arb

@@ -457,5 +457,14 @@
   "addTaskCancel": "Drop here to cancel",
   "@addTaskCancel": {
     "description": "The × left in the add button's place while it is dragged (D16)."
+  },
+  "taskMovedTo": "Moved to {day}",
+  "@taskMovedTo": {
+    "description": "Snack bar after a task was dragged onto another day in Upcoming (ADR 0033, D16).",
+    "placeholders": {
+      "day": {
+        "type": "String"
+      }
+    }
   }
 }