Enable Drag & Drop Reordering for Subtasks

Add the ability to drag and drop subtasks to easily change their order within a task. The updated order should be saved automatically and reflected consistently across all views (task view, list view, and client view if applicable).

Acceptance Criteria (optional):

Users can reorder subtasks via drag & drop
Order is persisted after refresh
Works on both desktop and mobile (if supported)
No impact on existing subtask data