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.Infrastructure/Services/StripeConnectService.cs

@@ -162,7 +162,6 @@ public class StripeConnectService : IStripeConnectService
         decimal invoiceTotal,
         decimal surchargeAmount,
         string currency,
-        string customerEmail,
         string invoiceNumber,
         int invoiceId)
     {
@@ -175,7 +174,6 @@ public class StripeConnectService : IStripeConnectService
             {
                 Amount = amountInCents,
                 Currency = currency.ToLower(),
-                ReceiptEmail = customerEmail,
                 Description = $"Invoice {invoiceNumber}",
                 Metadata = new Dictionary<string, string>
                 {
@@ -215,7 +213,6 @@ public class StripeConnectService : IStripeConnectService
         decimal depositAmount,
         decimal surchargeAmount,
         string currency,
-        string customerEmail,
         string quoteNumber,
         int quoteId)
     {
@@ -228,7 +225,6 @@ public class StripeConnectService : IStripeConnectService
             {
                 Amount = amountInCents,
                 Currency = currency.ToLower(),
-                ReceiptEmail = customerEmail,
                 Description = $"Deposit for quote {quoteNumber}",
                 Metadata = new Dictionary<string, string>
                 {