Fix Task Board Filters and Counters

Users report that the task board application exhibits several bugs when adding tasks, switching status filters, and maintaining the task counter accurate.

Requirements

• Display the count of currently visible tasks inside the element with data-testid task-counter in the format Showing X tasks (or Showing 1 task when exactly 1 task is visible).
• The list of tasks should display items fetched from /api/tasks upon initial page load.
• Submitting the task form with a non-empty title adds a new task with status pending to the list. Clear the input field after adding.
• If submitting an empty or whitespace-only task title, display an error message Task title cannot be empty in the form's error container with class error-message and do not add a task.
• Clicking status filter buttons (All, Pending, Completed) filters the displayed tasks accordingly. The active filter button must have the class active and aria-pressed attribute set to true.
• Toggling a task's checkbox updates its completed state, updates the filter view if a specific filter is active, and recalculates the counter accurately.
• Deleting a task removes it from the list and updates the counter immediately.

Reported bugs

1. When switching filters (e.g. to Pending), checking off a pending task keeps it displayed until the filter tab is re-clicked, and the task counter shows the wrong count.
2. When adding a new task while a filter like Completed is active, the newly created task (which is pending) immediately appears in the list even though the active filter is set to Completed.
3. Attempting to add an empty task shows no validation error, and clearing invalid text after an error retains the old error message.