Add BillingEmail field for commercial customers; support comma-separated multi-email

- Customer entity + DTO: new BillingEmail field (accounting/invoicing address)
- Email fields now accept comma-separated lists; DTO validates each address individually
- NotificationService: SendToEmailListAsync helper fans out to all addresses in a list;
  NotifyQuoteSentAsync accepts optional overrideEmail so staff can send to an ad-hoc address
- Migration: AddCustomerBillingEmail
- Customer Create/Edit/Details views updated to show Billing Email field
- customer-billing-email.js: client-side helpers for billing email input

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

src/PowderCoating.Application/Interfaces/INotificationService.cs

@@ -9,7 +9,7 @@ public interface INotificationService
     /// Notify when a quote is created/sent. Handles both registered customers and prospects.
     /// Optionally attaches the quote PDF to the email.
     /// </summary>
-    Task NotifyQuoteSentAsync(Quote quote, byte[]? pdfAttachment = null, string? pdfFilename = null);
+    Task NotifyQuoteSentAsync(Quote quote, byte[]? pdfAttachment = null, string? pdfFilename = null, string? overrideEmail = null);
 
     /// <summary>
     /// Sends the quote approval link to the customer via SMS.
@@ -58,7 +58,7 @@ public interface INotificationService
     /// Notify customer when an invoice has been sent.
     /// Optionally includes an online payment link in the email body.
     /// </summary>
-    Task NotifyInvoiceSentAsync(Invoice invoice, byte[]? pdfAttachment = null, string? pdfFilename = null, string? paymentUrl = null);
+    Task NotifyInvoiceSentAsync(Invoice invoice, byte[]? pdfAttachment = null, string? pdfFilename = null, string? paymentUrl = null, string? overrideEmail = null);
 
     /// <summary>
     /// Notify customer (internal) when a payment has been recorded on an invoice.