fix: make email button style consistent with other contact buttons

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

index.html

@@ -194,7 +194,7 @@
       <h2 class="section-title">Contact</h2>
       <p class="contact-desc">궁금한 점이 있으시면 편하게 연락해 주세요.</p>
       <div class="contact-links">
-        <a href="mailto:98kimsungmin@naver.com" class="btn btn-primary">이메일</a>
+        <a href="mailto:98kimsungmin@naver.com" class="btn">이메일</a>
         <a href="https://github.com/tolelom" class="btn" target="_blank" rel="noopener">GitHub</a>
         <a href="https://solved.ac/profile/tolelom" class="btn" target="_blank" rel="noopener">Solved.ac</a>
       </div>