fix(GRO-766): fix portal mobile overflow at 390px viewport

- CustomerPortal.tsx: change main from overflow-x-hidden to overflow-hidden
  to properly clip child overflow in both axes
- BillingPayments.tsx: add overflow-x-auto to tab button row so long
  button labels scroll instead of causing page-level overflow
- PetProfiles.tsx: already has overflow-x-auto on tab row — no change needed

Discovered in UAT by Shedward (DEF-2 and DEF-3 on GRO-754).

Co-Authored-By: Paperclip <noreply@paperclip.ing>

apps/web/src/portal/sections/BillingPayments.tsx

@@ -130,7 +130,7 @@ function BillingPaymentsInner({ sessionId, readOnly }: BillingPaymentsProps) {
         </div>
       )}
 
-      <div className="flex gap-2 flex-wrap">
+      <div className="flex gap-2 flex-wrap overflow-x-auto">
         {([
           { id: "invoices" as const, label: "Invoices", icon: DollarSign },
           { id: "payment" as const, label: "Payment Methods", icon: CreditCard },