feat: add VAT ID verify button and company info display

PlanSummary now has a Verify button for VAT ID instead of
auto-verifying on input. Shows company name and address from
VIES on successful verification, with warning on invalid IDs.

lib/api/billing.ts

@@ -109,6 +109,8 @@ export interface VATResult {
   total_amount: string
   vat_exempt: boolean
   vat_reason: string
+  company_name?: string
+  company_address?: string
 }
 
 export interface CalculateVATParams {