feat: payment method selector with foldable card form

Checkout shows payment method tiles (Card, Bancontact, iDEAL,
Apple Pay, SEPA DD, Bank Transfer). Card selection expands the
embedded form; other methods redirect to Mollie hosted checkout
with the method pre-selected.

lib/api/billing.ts

@@ -74,6 +74,7 @@ export interface CreateCheckoutParams {
   limit: number
   country: string
   vat_id?: string
+  method?: string
 }
 
 export async function createCheckoutSession(params: CreateCheckoutParams): Promise<{ url: string }> {