Center legend content and container on page

- Add text-align: center to legend for centered content
- Change margin from 2rem 0 to 2rem auto to center the legend box horizontally

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: GearUnclear

checklist.html

@@ -237,9 +237,10 @@
             background: white;
             border-radius: 12px;
             padding: 1.5rem;
-            margin: 2rem 0;
+            margin: 2rem auto;
             box-shadow: 0 4px 15px rgba(0,0,0,0.1);
             border-left: 5px solid var(--accent-color);
+            text-align: center;
         }
 
         .legend h3 {