Add custom styles and spacer for Admin nav button

Introduced a spacer above the admin navigation link to enhance sidebar organization. Updated the styles for the admin navigation button, including hover and active states, to improve clarity and visual feedback when interacting with the button.

frontend/src/App.jsx

@@ -62,7 +62,9 @@ function Layout({ children }) {
             <span className="nav-label">Alerts</span>
           </NavLink>
           {me?.role === "admin" && (
-            <NavLink to="/admin/users" className={navClass}>
+            <>
+              <div className="sidebar-nav-spacer" aria-hidden="true" />
+              <NavLink to="/admin/users" className={({ isActive }) => `nav-btn admin-nav${isActive ? " active" : ""}`}>
                 <span className="nav-icon" aria-hidden="true">
                   <svg viewBox="0 0 24 24">
                     <path d="M12 12a4 4 0 1 0 0-8 4 4 0 0 0 0 8zm-7 8a7 7 0 0 1 14 0" />
@@ -70,6 +72,7 @@ function Layout({ children }) {
                 </span>
                 <span className="nav-label">Admin</span>
               </NavLink>
+            </>
           )}
         </nav>
         <div className="profile">

frontend/src/styles.css

@@ -69,6 +69,13 @@ a {
   gap: 9px;
 }
 
+.sidebar-nav-spacer {
+  height: 12px;
+  margin: 2px 0 4px;
+  border-top: 1px solid #23406a;
+  opacity: 0.9;
+}
+
 .nav-btn {
   position: relative;
   display: flex;
@@ -107,6 +114,33 @@ a {
   background: linear-gradient(180deg, #74e8ff, #25bdf3);
 }
 
+.nav-btn.admin-nav {
+  border-color: #5b4da1;
+  background: linear-gradient(180deg, #1c2a58, #18224a);
+  color: #d8d5ff;
+}
+
+.nav-btn.admin-nav .nav-icon {
+  border-color: #7060c2;
+  background: linear-gradient(180deg, #2a2e66, #212854);
+}
+
+.nav-btn.admin-nav:hover {
+  border-color: #8b7ce0;
+  background: linear-gradient(180deg, #24316a, #1f2a5a);
+}
+
+.nav-btn.admin-nav.active {
+  border-color: #b9a7ff;
+  box-shadow: inset 0 0 0 1px #b9a7ff69, 0 8px 20px #1a143a8a;
+  background: linear-gradient(180deg, #332d73, #27235f);
+  color: #f4eeff;
+}
+
+.nav-btn.admin-nav.active::before {
+  background: linear-gradient(180deg, #ddd2ff, #b9a7ff);
+}
+
 .nav-icon {
   width: 28px;
   height: 28px;