Move payment settings to Brand tab in Settings

- Integrated PaymentSettingsForm into SettingsClient Brand tab
- Added paymentSettings prop to SettingsClient and Settings page
- Redirected /admin/settings/payments to /admin/settings#brand
- Updated PaymentSettingsForm to use admin CSS variables for styling
- Fixed colors to match admin design system (no dark backgrounds)

src/components/admin/AdminHeader.tsx

@@ -31,9 +31,6 @@ type SettingsItem = {
 
 const SETTINGS_ITEMS: SettingsItem[] = [
   { href: "/admin/settings", label: "General", description: "Brand name, logo, timezone" },
-  { href: "/admin/settings/brand", label: "Brand", description: "Brand profile and preferences" },
-  { href: "/admin/users", label: "Users & Permissions", description: "Team members and access roles" },
-  { href: "/admin/settings/integrations", label: "Integrations", description: "Stripe, Square, Resend, Twilio" },
   { href: "/admin/settings/apps", label: "Add-ons", description: "Enable and manage feature add-ons" },
   { href: "/admin/settings/billing", label: "Billing & Plans", description: "Subscription, invoices, usage" },
   { href: "/admin/settings/payments", label: "Payments", description: "Stripe, Square, payment methods" },