[미리보기] 버튼 상태 저장, 앱 실행시 직전 상태 적용

src/renderer/index.html

@@ -31,7 +31,7 @@
         <button class="sort-btn" id="btn-sort-date">날짜순</button>
       </div>
       <div class="separator"></div>
-      <button class="btn btn-info active" id="btn-preview-toggle">미리보기</button>
+      <button class="btn btn-info" id="btn-preview-toggle">미리보기</button>
     </nav>
 
     <form id="format-bar">