feat: improve button style and functionality

layouts/_default/single.html

@@ -2,7 +2,9 @@
 
 {{ define "main" }}
     <div class="coc-box" data-raw-content="{{ .RawContent }}">
-        <button class="copy-btn" onclick="copyCoC()">[copy]</button>
+        <button class="copy-btn" onclick="copyCoC()" title="Copy to clipboard">
+            <img src="/markdown-icon.svg" alt="Copy" class="copy-icon">
+        </button>
         {{ .Content }}
     </div>
     <p class="back-link"><a href="/">← Back to latest version</a> | <a href="/versions/">All versions</a></p>