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/style.css

@@ -61,6 +61,19 @@
   color: #173a27;
 }
 
+.sidebar-brand-head {
+  display: flex;
+  align-items: center;
+  gap: 8px;
+}
+
+.sidebar-brand-mark {
+  width: 26px;
+  height: 26px;
+  border-radius: 7px;
+  flex-shrink: 0;
+}
+
 .sidebar-brand strong {
   display: block;
   font-size: 16px;
@@ -309,12 +322,11 @@ a {
   width: 32px;
   height: 32px;
   border-radius: 9px;
-  display: grid;
-  place-items: center;
-  background: linear-gradient(180deg, #3d8654 0%, #245c3a 100%);
-  color: #fff;
-  font-weight: 700;
-  box-shadow: inset 0 1px 0 rgba(255, 255, 255, 0.24);
+  display: inline-block;
+  background: url('/appicon.png') center / cover no-repeat;
+  color: transparent;
+  font-size: 0;
+  box-shadow: inset 0 0 0 1px rgba(255, 255, 255, 0.2);
 }
 
 .app-brand-copy {