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.Core/Entities/Customer.cs

@@ -6,6 +6,7 @@ public class Customer : BaseEntity
     public string? ContactFirstName { get; set; }
     public string? ContactLastName { get; set; }
     public string? Email { get; set; }
+    public string? BillingEmail { get; set; }  // Accounting/invoicing email for commercial customers
     public string? Phone { get; set; }
     public string? MobilePhone { get; set; }
     public string? Address { get; set; }