Add dynamic task-name filter to Todo list UI

[Copilot]

FR-03 requires users to quickly find tasks in the current list via a text filter located at the top of the app. This change adds real-time title filtering without altering backend APIs.

• UI: top-level filter input

  • Added #taskFilter above the creation form in frontend/index.html.
  • Included accessible labeling and search-oriented placeholder text.

• Rendering flow: client-side dynamic filtering

  • Introduced currentTasks as the in-memory source of truth for the loaded list.
  • Added renderTasks() to centralize list rendering and apply case-insensitive title matching.
  • Wired taskFilter to input events for immediate updates while typing.
  • Updated task creation flow to append to currentTasks and re-render through the same path.

• Styling: visual consistency with existing themes

  • Added #taskFilter styles in frontend/style.css using existing theme variables (--input-bg, --text-color, focus shadow tokens), preserving light/dark behavior.

function renderTasks() {
  const filterValue = taskFilter.value.trim().toLowerCase();
  taskList.innerHTML = '';
  currentTasks
    .filter(task => task.title.toLowerCase().includes(filterValue))
    .forEach(addTaskToDOM);
}

taskFilter.addEventListener('input', renderTasks);