Update favicon, enhance sidebar branding, and improve CSS styles

- Replaced the favicon with a new app icon and added a shortcut icon for better branding.
- Enhanced the sidebar branding in ChatView.vue by introducing a logo element for improved visual appeal.
- Updated CSS styles to support new sidebar layout and icon display, ensuring a cohesive design.

These changes collectively improve the user interface and branding consistency across the application.

client/src/views/ChatView.vue

@@ -4,7 +4,10 @@
       <div class="auth-main-layout">
         <aside class="app-sidebar">
           <div class="sidebar-brand">
-            <strong>SingleChat</strong>
+            <div class="sidebar-brand-head">
+              <span class="app-brand-mark sidebar-brand-mark" aria-hidden="true">S</span>
+              <strong>SingleChat</strong>
+            </div>
             <span>Online Chat</span>
           </div>