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/DTOs/Customer/CustomerDtos.cs

@@ -9,6 +9,7 @@ public class CustomerDto
     public string? ContactFirstName { get; set; }
     public string? ContactLastName { get; set; }
     public string? Email { get; set; }
+    public string? BillingEmail { get; set; }
     public string? Phone { get; set; }
     public string? MobilePhone { get; set; }
     public string? Address { get; set; }
@@ -52,10 +53,13 @@ public class CreateCustomerDto : IValidatableObject
     public string? ContactLastName { get; set; }
 
     [Display(Name = "Email")]
-    [EmailAddress(ErrorMessage = "Please enter a valid email address")]
-    [StringLength(200)]
+    [StringLength(1000)]
     public string? Email { get; set; }
 
+    [Display(Name = "Billing / Accounting Email")]
+    [StringLength(1000)]
+    public string? BillingEmail { get; set; }
+
     [Display(Name = "Phone")]
     [Phone(ErrorMessage = "Please enter a valid phone number")]
     [StringLength(20)]
@@ -143,6 +147,33 @@ public class CreateCustomerDto : IValidatableObject
                 "Please provide at least one contact method (Email or Phone)",
                 new[] { nameof(Email), nameof(Phone) });
         }
+
+        // Validate each address in comma-separated email fields
+        foreach (var addr in SplitEmails(Email))
+        {
+            if (!IsValidEmail(addr))
+                yield return new ValidationResult(
+                    $"'{addr}' is not a valid email address.",
+                    new[] { nameof(Email) });
+        }
+        foreach (var addr in SplitEmails(BillingEmail))
+        {
+            if (!IsValidEmail(addr))
+                yield return new ValidationResult(
+                    $"'{addr}' is not a valid email address.",
+                    new[] { nameof(BillingEmail) });
+        }
+    }
+
+    private static IEnumerable<string> SplitEmails(string? value) =>
+        string.IsNullOrWhiteSpace(value)
+            ? []
+            : value.Split(',', StringSplitOptions.RemoveEmptyEntries | StringSplitOptions.TrimEntries);
+
+    private static bool IsValidEmail(string email)
+    {
+        try { _ = new System.Net.Mail.MailAddress(email); return true; }
+        catch { return false; }
     }
 }
 

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.