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/wwwroot/js/customer-billing-email.js

@@ -0,0 +1,12 @@
+(function () {
+    var select = document.getElementById('IsCommercial');
+    var row = document.getElementById('billingEmailRow');
+    if (!select || !row) return;
+
+    function toggle() {
+        row.style.display = select.value === 'true' ? '' : 'none';
+    }
+
+    toggle();
+    select.addEventListener('change', toggle);
+})();