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"
 }

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)."
   }
 }

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

apps/todo/halkyon_todo_flutter/lib/l10n/app_localizations_de.dart

@@ -707,4 +707,10 @@ class AppLocalizationsDe extends AppLocalizations {
 
   @override
   String get deadlineTitle => 'Frist';
+
+  @override
+  String get orderChanged => 'Reihenfolge geändert';
+
+  @override
+  String get addTaskCancel => 'Hier ablegen zum Abbrechen';
 }

apps/todo/halkyon_todo_flutter/lib/l10n/app_localizations_en.dart

@@ -706,6 +706,12 @@ class AppLocalizationsEn extends AppLocalizations {
 
   @override
   String get deadlineTitle => 'Deadline';
+
+  @override
+  String get orderChanged => 'Order changed';
+
+  @override
+  String get addTaskCancel => 'Drop here to cancel';
 }
 
 /// The translations for English, as used in the United Kingdom (`en_GB`).