Fix subtask duplication when moving tasks between backends

[alainm23]

Fixes visual duplication of subtasks when moving tasks with children between different backend types (e.g., LOCAL to Todoist).

Changes:

• Fixed subtasks being added to both parent and project when moving between backends
• Reduced toast notifications to show only once per move operation instead of per subtask

The issue occurred because add_final_duplicate_item was adding all items directly to the project, regardless of parent relationship. Now subtasks are correctly added to their parent item only.

Fixes: #1426