Add invoice SMS notifications and customer intake kiosk

Invoice SMS:
- Send Invoice modal now prompts Email/SMS/Both based on customer contact data
- New /invoice/{token} customer-facing view page with full line items and pay button
- PublicViewToken (permanent) added to Invoice; separate from expiring PaymentLinkToken
- InvoiceSent SMS default template added; customizable via Notification Templates settings
- {{viewUrl}} placeholder documented in template editor

Customer Intake Kiosk:
- Tablet kiosk flow: Contact → Job → Terms/Signature → Confirmation
- Remote link mode for off-site customers (lighter form, no signature)
- KioskHub (AllowAnonymous SignalR) for staff-to-tablet push without login
- Staff activates tablet via cookie; sends remote link manually
- Submitted sessions create Customer + Job automatically; fires in-app notification

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

src/PowderCoating.Core/Entities/Invoice.cs

@@ -28,6 +28,13 @@ public class Invoice : BaseEntity
     public decimal GiftCertificateRedeemed { get; set; }   // Sum of gift certificate redemptions
     public decimal BalanceDue => Total - AmountPaid - CreditApplied - GiftCertificateRedeemed;
 
+    /// <summary>
+    /// Permanent public token for the customer-facing invoice view page (/invoice/{token}).
+    /// Generated when the invoice is first sent (regardless of Stripe status) and never expires.
+    /// Distinct from PaymentLinkToken which is Stripe-gated and expires in 5 days.
+    /// </summary>
+    public string? PublicViewToken { get; set; }
+
     // Online payments (Stripe Connect)
     public OnlinePaymentStatus OnlinePaymentStatus { get; set; } = OnlinePaymentStatus.NotApplicable;
     public string? PaymentLinkToken { get; set; }           // Signed token for /pay/{token}