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.Web/Views/Customers/Edit.cshtml

@@ -78,7 +78,7 @@
                             </div>
                             <div class="col-md-6">
                                 <label asp-for="Email" class="form-label">Email</label>
-                                <input asp-for="Email" type="email" class="form-control" placeholder="name@example.com" />
+                                <input asp-for="Email" type="email" multiple class="form-control" placeholder="name@example.com (comma-separate multiple)" />
                                 <span asp-validation-for="Email" class="text-danger"></span>
                             </div>
                             <div class="col-md-3">
@@ -91,6 +91,14 @@
                                 <input asp-for="MobilePhone" type="tel" class="form-control" placeholder="(555) 123-4567" />
                                 <span asp-validation-for="MobilePhone" class="text-danger"></span>
                             </div>
+                            <div class="col-md-6" id="billingEmailRow" style="display:none;">
+                                <label asp-for="BillingEmail" class="form-label">Billing / Accounting Email
+                                    <span class="text-muted fw-normal">(invoices sent here)</span>
+                                </label>
+                                <input asp-for="BillingEmail" type="email" multiple class="form-control" placeholder="accounting@company.com (comma-separate multiple)" />
+                                <span asp-validation-for="BillingEmail" class="text-danger"></span>
+                                <div class="form-text">When set, invoices are emailed here instead of the contact email.</div>
+                            </div>
                         </div>
                     </div>
 
@@ -436,4 +444,5 @@
 
 @section Scripts {
     <partial name="_ValidationScriptsPartial" />
+    <script src="~/js/customer-billing-email.js"></script>
 }