feat(frontend): add reusable admin sidebar component

- Create AdminSidebar component for admin panel navigation
- Support active state highlighting based on currentPath
- Consistent navigation across all admin pages
- Replaces duplicated sidebar markup in admin templates

templates/admin_sidebar.templ

@@ -0,0 +1,29 @@
+package templates
+
+templ AdminSidebar(user User, currentPath string) {
+	<aside class="w-64 border-r" style="background-color: var(--bg-secondary); border-color: var(--border)">
+		<div class="p-6">
+			<h2 class="text-lg font-semibold mb-6" style="color: var(--text-primary)">Admin Panel</h2>
+			<nav class="space-y-2">
+				<a href="/admin"
+				   class={"block px-4 py-2 rounded-lg " +
+					   ("bg-accent text-bg-primary" if currentPath == "/admin" else "hover:opacity-80")}
+				   style="color: var(--text-primary)">
+					🏠 Dashboard
+				</a>
+				<a href="/admin/users"
+				   class={"block px-4 py-2 rounded-lg " +
+					   ("bg-accent text-bg-primary" if currentPath == "/admin/users" else "hover:opacity-80")}
+				   style="color: var(--text-primary)">
+					👤 User Administration
+				</a>
+				<a href="/admin/library"
+				   class={"block px-4 py-2 rounded-lg " +
+					   ("bg-accent text-bg-primary" if currentPath == "/admin/library" else "hover:opacity-80")}
+				   style="color: var(--text-primary)">
+					📚 Library Management
+				</a>
+			</nav>
+		</div>
+	</aside>
+}