Implement "Clear Completed" functionality

[MustaphaBakoji]

Overview:
As users finish multiple tasks, the list can become cluttered with "checked" items. We need a way for users to bulk-delete only the tasks that are marked as completed, without affecting their active To-Do list.

Proposed Changes:

UI: Add a "Clear Completed" button, ideally near the footer or the top of the task list.

Logic: Create a function that filters the task array/database to remove items where status === 'completed' (or your equivalent boolean).

Validation: Show a simple confirmation toast or tooltip once the tasks are cleared.

Acceptance Criteria:

Button is visible only when at least one task is completed.

Clicking the button removes all completed tasks from state/LocalStorage.

The UI updates immediately to show only pending tasks.