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/App.tsx

@@ -45,6 +45,7 @@ const CustomerDashboard = React.lazy(() => import('./pages/customer/CustomerDash
 const CustomerSupport = React.lazy(() => import('./pages/customer/CustomerSupport'));
 const ResourceDashboard = React.lazy(() => import('./pages/resource/ResourceDashboard'));
 const BookingPage = React.lazy(() => import('./pages/customer/BookingPage'));
+const CustomerBilling = React.lazy(() => import('./pages/customer/CustomerBilling'));
 const TrialExpired = React.lazy(() => import('./pages/TrialExpired'));
 const Upgrade = React.lazy(() => import('./pages/Upgrade'));
 
@@ -537,7 +538,7 @@ const AppContent: React.FC = () => {
           >
             <Route path="/" element={<CustomerDashboard />} />
             <Route path="/book" element={<BookingPage />} />
-            <Route path="/payments" element={<Payments />} />
+            <Route path="/payments" element={<CustomerBilling />} />
             <Route path="/support" element={<CustomerSupport />} />
             <Route path="/profile" element={<ProfileSettings />} />
             <Route path="/verify-email" element={<VerifyEmail />} />