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/DevQuickLogin.tsx

@@ -88,14 +88,14 @@ export function DevQuickLogin({ embedded = false }: DevQuickLoginProps) {
   const handleQuickLogin = async (user: TestUser) => {
     setLoading(user.email);
     try {
-      // Call token auth API - username field contains email since we use email as username
-      const response = await apiClient.post('/auth-token/', {
-        username: user.email,
+      // Call custom login API that supports email login
+      const response = await apiClient.post('/auth/login/', {
+        email: user.email,
         password: user.password,
       });
 
       // Store token in cookie (use 'access_token' to match what client.ts expects)
-      setCookie('access_token', response.data.token, 7);
+      setCookie('access_token', response.data.access, 7);
 
       // Clear any existing masquerade stack - this is a fresh login
       localStorage.removeItem('masquerade_stack');