feat: add comprehensive demo application with custom payment UI

- Custom test payment UI with modern Tailwind CSS design
- Payment method selection (iDEAL, Credit Card, PayPal, Apple Pay, Bank Transfer)
- Test scenario selection (6 scenarios: success, delayed, cancelled, declined, expired, pending)
- Real-time payment status polling
- Success and failure result pages with payment details
- Interactive demo homepage at root path
- Sample data seeding (customers, invoices)
- Customers collection with auto-sync to invoices
- Comprehensive documentation (README.md, DEMO_GUIDE.md)
- Proper cursor styles for all interactive elements

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

dev/app/api/demo/create-payment/route.ts

@@ -0,0 +1,56 @@
+import configPromise from '@payload-config'
+import { getPayload } from 'payload'
+
+export async function POST(request: Request) {
+  try {
+    const payload = await getPayload({
+      config: configPromise,
+    })
+
+    const body = await request.json()
+    const { amount, currency, description } = body
+
+    if (!amount || !currency) {
+      return Response.json(
+        { success: false, error: 'Amount and currency are required' },
+        { status: 400 }
+      )
+    }
+
+    // Create a payment using the test provider
+    const payment = await payload.create({
+      collection: 'payments',
+      data: {
+        provider: 'test',
+        amount,
+        currency,
+        description: description || 'Demo payment',
+        status: 'pending',
+        metadata: {
+          source: 'demo-ui',
+          createdAt: new Date().toISOString(),
+        },
+      },
+    })
+
+    return Response.json({
+      success: true,
+      payment: {
+        id: payment.providerId, // Use the test provider ID for the UI
+        paymentId: payment.id,
+        amount: payment.amount,
+        currency: payment.currency,
+        description: payment.description,
+      },
+    })
+  } catch (error) {
+    console.error('Failed to create payment:', error)
+    return Response.json(
+      {
+        success: false,
+        error: error instanceof Error ? error.message : 'Failed to create payment',
+      },
+      { status: 500 }
+    )
+  }
+}