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/ConnectOnboardingEmbed.tsx

@@ -70,7 +70,7 @@ const ConnectOnboardingEmbed: React.FC<ConnectOnboardingEmbedProps> = ({
             colorDanger: '#df1b41',
             fontFamily: 'system-ui, -apple-system, sans-serif',
             fontSizeBase: '14px',
-            spacingUnit: '4px',
+            spacingUnit: '12px',
             borderRadius: '8px',
           },
         },