feat(todo): quick add's keys, sub-projects' colours, a task's menu on a right click, and the completed tasks deleted at once

The developer's list of 9 Nov 2026, Todo's part:

- Quick add: the down arrow and Tab move into the description, with its
  cursor (halkyon_editor's fix); Escape closes the projects or labels
  suggested first, then closes quick add, what was typed discarded; the
  project's chip shows the project's colour.
- A project without a colour of its own shows its parent's, the nearest
  ancestor's with one (colorOfProject, TodoCubit.colorOf), wherever a
  project's colour shows; the project editor's "Same colour as its parent"
  keeps it so, and the parent picker shows each project in its colour. A
  project of its own always has one.
- The task pane's x closes it, always; Close left the ⋮ menu.
- A task's row opens its menu on a right click: Mark as done, Date, Move
  to, Labels, Priority, Deadline, Duplicate and Delete, the selection's
  actions for that one task.
- Delete completed tasks, in the ⋮ of the Inbox, Today, Upcoming and a
  project, with Undo: the tasks leave the list at once and are deleted
  once the snack bar goes without its Undo (halkyon_ui's showUndoSnackBar
  gains onKept).

Whats-New: Quick add: the arrow keys and Tab reach the description, and Escape closes it without saving.
Whats-New-De: Schnell hinzufügen: Pfeiltasten und Tab erreichen die Beschreibung, und Escape schließt es, ohne zu speichern.
Whats-New: Sub-projects take their parent's colour unless they have their own.
Whats-New-De: Unterprojekte übernehmen die Farbe ihres übergeordneten Projekts, wenn sie keine eigene haben.
Whats-New: A right click on a task opens its menu, with Move to and more; Delete completed tasks clears a list, with Undo.
Whats-New-De: Ein Rechtsklick auf eine Aufgabe öffnet ihr Menü, mit Verschieben nach und mehr; Erledigte Aufgaben löschen räumt eine Liste auf, mit Rückgängig.
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

@@ -282,6 +282,10 @@
       }
     }
   },
+  "projectInheritColor": "Same colour as its parent",
+  "@projectInheritColor": {
+    "description": "In a sub-project's editor: whether it shows in its parent's colour rather than one of its own (the developer's review, 9 Nov 2026)."
+  },
   "projectParent": "Parent project",
   "@projectParent": {
     "description": "A project's editor (ADR 0033, D28), as Todoist's Add project."
@@ -598,7 +602,6 @@
   "taskDuplicated": "Task duplicated",
   "subTasksHideDone": "Hide done sub-tasks",
   "subTasksShowDone": "Show done sub-tasks",
-  "taskClose": "Close",
   "taskPath": "Where the task is",
   "weekToday": "Back to today",
   "monthPrevious": "Previous month",
@@ -1246,5 +1249,18 @@
         "type": "String"
       }
     }
+  },
+  "tasksDeleteCompleted": "Delete completed tasks",
+  "@tasksDeleteCompleted": {
+    "description": "A list's menu: deletes its done tasks, with an Undo (the developer's review, 9 Nov 2026)."
+  },
+  "tasksCompletedDeleted": "{count, plural, =1{1 completed task deleted} other{{count} completed tasks deleted}}",
+  "@tasksCompletedDeleted": {
+    "description": "The snack bar after Delete completed tasks, with its Undo.",
+    "placeholders": {
+      "count": {
+        "type": "int"
+      }
+    }
   }
 }