fix(billing): widen tracking exception catch so post-payment page never crashes

`$account->stripe()` can throw `\Stripe\Exception\InvalidArgumentException`
(descends from PHP's `InvalidArgumentException`, not `ApiErrorException`)
when the Stripe key is missing/malformed. Since the conversion data is
purely for tracking, any failure must degrade silently — not break the
post-payment success page.

app/Http/Controllers/App/BillingController.php

@@ -11,8 +11,8 @@
 use Illuminate\Support\Facades\Gate;
 use Inertia\Inertia;
 use Inertia\Response;
-use Stripe\Exception\ApiErrorException as StripeApiErrorException;
 use Symfony\Component\HttpFoundation\Response as SymfonyResponse;
+use Throwable;
 
 class BillingController extends Controller
 {
@@ -101,7 +101,7 @@ private function buildConversionData(Account $account, string $sessionId): ?arra
                 $sessionId,
                 ['expand' => ['line_items.data.price']],
             );
-        } catch (StripeApiErrorException) {
+        } catch (Throwable) {
             return null;
         }