revert: restore GitHub + Email buttons in hero

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

index.html

@@ -42,8 +42,8 @@
       </div>
       <div class="hero-links">
         <a href="https://github.com/tolelom" class="btn btn-primary" target="_blank" rel="noopener">GitHub</a>
+        <a href="mailto:98kimsungmin@naver.com" class="btn">Email</a>
       </div>
-      <p class="hero-email"><a href="mailto:98kimsungmin@naver.com">98kimsungmin@naver.com</a></p>
     </section>
 
     <section id="about" class="section">