fix: add missing title for Issue 31 in archive 12

The Issue 31 heading only showed the issue number without the actual
title. Extracted the number into a span and added the full title.

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

Author: vinta

website/archives/12/index.html

@@ -260,7 +260,8 @@ <h4 class="item__footer">
           <li class="item">
             <a href="/issues/31/#start">
               <div class="item__body">
-                <h2 class="item__heading">Issue 31</h2>
+                <span class="item__issue-number">Issue 31</span>
+                <h2 class="item__heading">「我是個壞女孩，我需要被懲罰」她咬著嘴唇說著，「很好」他邊說邊開始在她的筆電安裝 Windows 10 #阿宅的五十道陰影</h2>
                 <p class="item__title">
                   本期內容涵蓋 React, Django, HTTP/2, zsh, Web Workers,
                   JavaScript, CSS, npm, iOS, GitHub, Swift, Objective-C