feat(billing): Add customer billing page with payment method management

- Add CustomerBilling page for customers to view payment history and manage cards
- Create AddPaymentMethodModal with Stripe Elements for secure card saving
- Support both Stripe Connect and direct API payment modes
- Auto-set first payment method as default when no default exists
- Add dark mode support for Stripe card input styling
- Add customer billing API endpoints for payment history and saved cards
- Add stripe_customer_id field to User model for Stripe customer tracking

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

smoothschedule/payments/urls.py

@@ -33,6 +33,12 @@ from .views import (
     CreatePaymentIntentView,
     TerminalConnectionTokenView,
     RefundPaymentView,
+    # Customer billing
+    CustomerBillingView,
+    CustomerPaymentMethodsView,
+    CustomerSetupIntentView,
+    CustomerPaymentMethodDeleteView,
+    CustomerPaymentMethodDefaultView,
 )
 
 urlpatterns = [
@@ -71,4 +77,11 @@ urlpatterns = [
     path('payment-intents/', CreatePaymentIntentView.as_view(), name='create-payment-intent'),
     path('terminal/connection-token/', TerminalConnectionTokenView.as_view(), name='terminal-connection-token'),
     path('refunds/', RefundPaymentView.as_view(), name='create-refund'),
+
+    # Customer billing endpoints
+    path('customer/billing/', CustomerBillingView.as_view(), name='customer-billing'),
+    path('customer/payment-methods/', CustomerPaymentMethodsView.as_view(), name='customer-payment-methods'),
+    path('customer/setup-intent/', CustomerSetupIntentView.as_view(), name='customer-setup-intent'),
+    path('customer/payment-methods/<str:payment_method_id>/', CustomerPaymentMethodDeleteView.as_view(), name='customer-payment-method-delete'),
+    path('customer/payment-methods/<str:payment_method_id>/default/', CustomerPaymentMethodDefaultView.as_view(), name='customer-payment-method-default'),
 ]