refactor(admin): integrate reusable sidebar into admin pages

- Update admin.templ to use @AdminSidebar component
- Update admin_library.templ to use @AdminSidebar component
- Update admin_users.templ to use @AdminSidebar component
- Add sidebar layout wrapper to admin_users.templ (previously missing)
- Fix theme rendering to use user.Theme in all admin templates
- Remove duplicated sidebar markup across admin pages

templates/admin.templ

@@ -14,23 +14,7 @@ templ Admin(user User) {
         @Header(user, "/admin")
 
         <div class="flex min-h-screen" style="background-color: var(--bg-primary)">
-            <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" style="color: var(--text-primary)">
-                            🏠 Dashboard
-                        </a>
-                        <a href="/admin/users" class="block px-4 py-2 rounded-lg hover:opacity-80" style="color: var(--text-primary)">
-							👤 User Administration
-						</a>
-                        <a href="/admin/library" class="block px-4 py-2 rounded-lg hover:opacity-80" style="color: var(--text-primary)">
-                            📚 Library Management
-                        </a>
-                    </nav>
-                </div>
-            </aside>
-
+            @AdminSidebar(user, "/admin")
             <main class="flex-1 p-8">
                 <div class="max-w-4xl">
                     <div class="mb-8">