fix: block Stripe auto-fallback for non-US users (#2529)

## Summary
- The token fallback hook (`useTokenFallback`) auto-selects Stripe when
no crypto token has sufficient balance — this was bypassing the US-only
fiat gate from #2528
- Pass a no-op callback instead of `onStripeSelect` for non-US users so
fiat checkout is never auto-selected outside the US

## Test plan
- [ ] Non-US user with insufficient crypto balance should NOT
auto-switch to credit card
- [ ] US user with insufficient crypto balance should still
auto-fallback to Stripe

🪟 Made with [Lattice](https://github.com/Larkooo/lattice)

Author: Larkooo

packages/keychain/src/components/purchasenew/checkout/onchain/index.tsx

@@ -169,6 +169,8 @@ export function OnchainCheckout() {
     quantity,
   });
 
+  const isUS = countryCode === "US";
+
   const { isCheckingFallback } = useTokenFallback({
     controller,
     starterpackDetails: starterpackDetails as
@@ -184,7 +186,7 @@ export function OnchainCheckout() {
     isApplePaySelected,
     selectedPlatform,
     setSelectedToken,
-    onStripeSelect,
+    onStripeSelect: isUS ? onStripeSelect : () => {},
   });
 
   const globalDisabled = useMemo(() => {