chore: update design icon to match with other design tools

Author: abose

src/index.html

@@ -937,7 +937,7 @@
     <div id="centralControlBar" class="no-focus">
         <div class="ccb-group ccb-group-top">
             <a href="#" id="ccbCollapseEditorBtn" class="ccb-btn" title="Switch to Design Mode">
-                <i class="fa-solid fa-feather"></i>
+                <i class="fa-solid fa-pen-nib fa-rotate-90"></i>
             </a>
             <a href="#" id="ccbSidebarToggleBtn" class="ccb-btn" title="Toggle sidebar">
                 <i class="fa-solid fa-angles-left"></i>