Mark as Completed

User Story

As a user, I want to mark tasks as completed and have them sorted to focus on pending tasks.

Acceptance Criteria

• Implement checkboxes for marking tasks as completed.
• Apply a strikethrough effect to the text of completed tasks.
• Automatically sort tasks, placing uncompleted ones at the top.
• Display a dynamic counter (e.g., "2/4 todos completed") in the app's bottom right, visible only when tasks are present.