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/settings/ai/page.tsx

@@ -1,28 +1,9 @@
 import { redirect } from "next/navigation";
 import { getAdminUser } from "@/lib/admin-permissions";
-import { getAIProviderSettings } from "@/actions/integrations/ai-providers";
-import AIClient from "./AIClient";
-import AdminAccessDenied from "@/components/admin/AdminAccessDenied";
 
 export default async function AISettingsPage() {
   const adminUser = await getAdminUser();
-  if (!adminUser) return <AdminAccessDenied />;
+  if (!adminUser) redirect("/login");
 
-  const brandId = adminUser.brand_id ?? "";
-
-  const settings = await getAIProviderSettings(brandId);
-  const isConnected = !!settings.apiKey;
-
-  const brandName = "Brand"; // Note: resolved from adminUser.brand_id on the server; hardcoded fallback for settings UI
-
-  return (
-    <AIClient
-      isConnected={isConnected}
-      brandId={brandId}
-      brandName={brandName}
-      provider={settings.provider}
-      model={settings.model}
-      customEndpoint={settings.customEndpoint}
-    />
-  );
+  redirect("/admin/settings#ai");
 }