fix: settings page redesign - tabbed interface, no sidebar dropdown

- AdminSidebar: removed dropdown menu, flat nav links, no scrolling
  - Removed SETTINGS_SUB_LINKS array
  - Removed settingsOpen state and dropdown logic
  - Settings is now a simple link to /admin/settings
  - Nav uses overflow-hidden to prevent sidebar scroll

- SettingsClient: unified tabbed interface (like CommunicationsPage)
  - New tabs: General, Add-ons, Billing, Integrations, AI, Advanced
  - Uses AdminFilterTabs for navigation
  - Each tab shows a card with link to full settings page

- Redirected pages to /admin/settings#tab:
  - /admin/settings/apps → /admin/settings#addons
  - /admin/settings/integrations → /admin/settings#integrations
  - /admin/settings/ai → /admin/settings#ai
  - /admin/advanced → /admin/settings#advanced

All TypeScript checks pass.

src/app/admin/advanced/page.tsx

@@ -1,53 +1,9 @@
 import { redirect } from "next/navigation";
 import { getAdminUser } from "@/lib/admin-permissions";
-import { getBrands } from "@/actions/admin/users";
-import { getStripeConnectStatus } from "@/actions/stripe-connect";
-import AdvancedSettingsClient from "@/components/admin/AdvancedSettingsClient";
-import { PageHeader } from "@/components/admin/design-system";
-
-export const metadata = {
-  title: "Advanced Settings - Route Commerce Admin",
-  description: "Developer settings, API integrations, and advanced configurations",
-};
 
 export default async function AdvancedSettingsPage() {
   const adminUser = await getAdminUser();
   if (!adminUser) redirect("/login");
 
-  // Only admins can access advanced settings
-  if (!adminUser.can_manage_settings && adminUser.role !== "platform_admin") {
-    redirect("/admin");
-  }
-
-  const brandId = adminUser.brand_id ?? "";
-  const { brands } = await getBrands();
-  
-  // Get Stripe Connect status
-  const stripeConnect = brandId ? await getStripeConnectStatus(brandId) : null;
-
-  return (
-    <main className="min-h-screen bg-[var(--admin-bg)] px-6 py-10">
-      <div className="mx-auto max-w-4xl">
-        <PageHeader
-          breadcrumb={[
-            { label: "Admin", href: "/admin" },
-            { label: "Settings", href: "/admin/settings" },
-            { label: "Advanced" },
-          ]}
-          title="Advanced Settings"
-          subtitle="Developer settings, APIs, and integrations"
-          icon={
-            <svg className="h-6 w-6" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2" strokeLinecap="round" strokeLinejoin="round">
-              <path d="M9.813 15.904L9 18.75l-.813-2.846a4.5 4.5 0 00-3.09-3.09L2.25 12l2.846-.813a4.5 4.5 0 003.09-3.09L9 5.25l.813 2.846a4.5 4.5 0 003.09 3.09L15.75 12l-2.846.813a4.5 4.5 0 00-3.09 3.09zM18.259 8.715L18 9.75l-.259-1.035a3.375 3.375 0 00-2.455-2.456L14.25 6l1.036-.259a3.375 3.375 0 002.455-2.456L18 2.25l.259 1.035a3.375 3.375 0 002.456 2.456L21.75 6l-1.035.259a3.375 3.375 0 00-2.456 2.456zM16.894 20.567L16.5 21.75l-.394-1.183a2.25 2.25 0 00-1.423-1.423L13.5 18.75l1.183-.394a2.25 2.25 0 001.423-1.423l.394-1.183.394 1.183a2.25 2.25 0 001.423 1.423l1.183.394-1.183.394a2.25 2.25 0 00-1.423 1.423z"/>
-            </svg>
-          }
-        />
-        <AdvancedSettingsClient
-          brandId={brandId}
-          brands={brands}
-          stripeConnect={stripeConnect}
-        />
-      </div>
-    </main>
-  );
+  redirect("/admin/settings#advanced");
 }