feat(todo): a task's title field hints "Task name", as Todoist's

The title's placeholder in quick add and in a task's sheet read "What's
to be done?"; the developer asked for Todoist's plainer "Task name"
("Aufgabenname").

Whats-New: A task's title field now says "Task name".
Whats-New-De: Das Titelfeld einer Aufgabe zeigt jetzt „Aufgabenname“.

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

@@ -181,7 +181,7 @@
       }
     }
   },
-  "taskTitleHint": "What's to be done?",
+  "taskTitleHint": "Task name",
   "taskDescription": "Description",
   "taskDescriptionHint": "Description",
   "taskComplete": "Mark as done",