Payment hardening: InvariantCulture on JS literals, remove dead CustomerEmail

Razor numeric expressions emitted into JS literals (MAX_TOTAL,
SURCHARGE_VALUE) now use InvariantCulture, matching the pattern already
used on the deposit page. Without this, a server culture with comma
decimal separators would silently truncate values like 2.5% to 2.

CustomerEmail removed from PaymentPageViewModel and
DepositPaymentPageViewModel — it was populated from the DB on every
payment page load but never consumed after receipt_email was removed
from the Stripe PaymentIntent.

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

src/PowderCoating.Web/Views/Payment/Index.cshtml

@@ -116,9 +116,9 @@
     const STRIPE_PK  = '@Model.StripePublishableKey';
     const ACCOUNT_ID = '@Model.StripeAccountId';
     const TOKEN      = '@Model.Token';
-    const MAX_TOTAL  = @Model.TotalWithSurcharge.ToString("F2");
+    const MAX_TOTAL  = @Model.TotalWithSurcharge.ToString("F2", System.Globalization.CultureInfo.InvariantCulture);
     const SURCHARGE_TYPE  = '@Model.SurchargeType';
-    const SURCHARGE_VALUE = @Model.SurchargeValue.ToString("F4");
+    const SURCHARGE_VALUE = @Model.SurchargeValue.ToString("F4", System.Globalization.CultureInfo.InvariantCulture);
     const SUCCESS_URL = `/pay/${TOKEN}/success`;
 
     const stripe = Stripe(STRIPE_PK, { stripeAccount: ACCOUNT_ID });