feat: add admin pages for plans, users, and placeholders

- AdminPlans: full CRUD UI with cards, visibility toggle
- AdminUsers: search, suspend/unsuspend, grant Pro access
- AdminPaymentMethods: placeholder
- AdminPayments: placeholder
- AdminSettings: placeholder
- All routes wired in App.tsx
- Admin panel fully navigable

apps/web/src/components/admin/pages/AdminSettings.tsx

@@ -0,0 +1,12 @@
+export function AdminSettings() {
+  return (
+    <div>
+      <h1 className="text-3xl font-bold text-gray-900 dark:text-white mb-4">
+        App Settings
+      </h1>
+      <p className="text-gray-600 dark:text-gray-400">
+        Manage app configuration (Coming soon)
+      </p>
+    </div>
+  )
+}