Add instructions field to quiz and matching item types

Instructions are optional free text shown to the student above the questions/word bank.
Admin enters them in the item editor; stored in items.body for quiz and matching types.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

public/index.html

@@ -135,6 +135,7 @@
     <div class="item-card">
       <p class="item-kicker" id="quiz-kicker"></p>
       <h1 id="quiz-title"></h1>
+      <p id="quiz-instructions" class="quiz-instructions" hidden></p>
       <form id="quiz-form"></form>
       <div id="quiz-result" class="status-banner" hidden></div>
     </div>