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