settings: move dues config (Stripe + payment links) into its own Dues tab

The Stripe recurring-dues panel and the external payment-links panel were
rendered below the settings tab bar (outside SettingsClient), so they
floated under every tab. Add a Dues tab (Space / Integrations / Dues /
Webhooks) and render both panels there as standard tab cards. UI only.

components/settings/dues-payment-methods-panel.tsx

@@ -78,7 +78,7 @@ export function DuesPaymentMethodsPanel() {
   }
 
   return (
-    <div className="p-4 md:p-6 border-t border-border">
+    <div className="bg-card rounded border border-border p-6">
       <div className="max-w-2xl space-y-3">
         <h2 className="font-mono text-[10px] uppercase tracking-widest text-muted-foreground">
           Other ways to pay dues

components/settings/stripe-billing-panel.tsx

@@ -56,7 +56,7 @@ export function StripeBillingPanel({ spaceId }: { spaceId: string }) {
   }
 
   return (
-    <div className="p-4 md:p-6 border-t border-border">
+    <div className="bg-card rounded border border-border p-6">
       <div className="max-w-2xl space-y-3">
         <div className="flex items-center justify-between">
           <h2 className="font-mono text-[10px] uppercase tracking-widest text-muted-foreground">