Fix Stripe receipt_email + online payment surcharge and hardening

Remove receipt_email from PaymentIntent creation so customers can use
any email at Stripe checkout without a stored-email mismatch blocking
payment. Remove now-dead CustomerEmail from PaymentPageViewModel.

Fix surcharge payment input: amount field now represents the total the
customer pays (including fee); JS back-calculates base before sending
to server. Add InvariantCulture to numeric Razor→JS literals to prevent
comma-decimal cultures from truncating surcharge values.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/PowderCoating.Application/Interfaces/IStripeConnectService.cs

@@ -20,7 +20,6 @@ public interface IStripeConnectService
         decimal invoiceTotal,
         decimal surchargeAmount,
         string currency,
-        string customerEmail,
         string invoiceNumber,
         int invoiceId);
 
@@ -33,7 +32,6 @@ public interface IStripeConnectService
         decimal depositAmount,
         decimal surchargeAmount,
         string currency,
-        string customerEmail,
         string quoteNumber,
         int quoteId);
 }