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>

frontend/src/components/CreditPaymentForm.tsx

@@ -377,7 +377,7 @@ export const CreditPaymentModal: React.FC<CreditPaymentModalProps> = ({
                   colorText: '#1e293b',
                   colorDanger: '#dc2626',
                   fontFamily: 'system-ui, -apple-system, sans-serif',
-                  spacingUnit: '4px',
+                  spacingUnit: '12px',
                   borderRadius: '8px',
                 },
               },