Separate theme and quick login; remove login button

Add a visual divider between theme and quick login, remove the header login button, and prevent end-users from seeing the login config modal by default.

Co-authored-by: Cursor <cursoragent@cursor.com>

index.html

@@ -70,6 +70,7 @@
           <button class="btn" id="btnTheme" type="button" aria-pressed="false" title="테마 전환">
             테마
           </button>
+          <span class="divider" aria-hidden="true"></span>
           <div class="user" id="user" hidden>
             <span class="user-dot" aria-hidden="true"></span>
             <span class="user-text" id="userText"></span>
@@ -88,7 +89,6 @@
               </button>
             </div>
           </div>
-          <button class="btn" id="btnLogin" type="button">로그인</button>
           <button class="btn" id="btnLogout" type="button" hidden>로그아웃</button>
         </div>
       </div>