chore: add spacing and mobile-only note above QR Code Test button

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

Author: vveerrgg

docs/test.html

@@ -385,7 +385,8 @@ <h1>NostrKey</h1>
                 </button>
             </div>
 
-            <div class="section" style="margin-top: 8px">
+            <div class="section" style="margin-top: 32px; border-top: 1px solid #334155; padding-top: 16px">
+                <p style="font-size: 0.75rem; color: #94a3b8; text-align: center; margin-bottom: 8px;">QR code scanning is only available in the mobile apps (iOS & Android).</p>
                 <a href="test-qr-codes.html" class="btn btn-method" style="text-decoration:none;text-align:center;display:block">
                     &#128247; QR Code Test
                 </a>