Build and use reusable interactive exercise islands for this Lessons site — multiple-choice and multi-answer questions, quizzes, and concept→definition matching/linking exercises. Covers using the existing components (MCQ, Quiz) AND authoring new reusable React islands the right way (barrel export, design tokens, a11y, i18n label props). Use when the user asks to "add a quiz/exercise", "multi-select question", "match concepts to definitions", "make a reusable exercise component", or to extend the component library.