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/tailwind.config.ts

@@ -0,0 +1,14 @@
+import type { Config } from 'tailwindcss'
+
+const config: Config = {
+  content: [
+    './app/**/*.{js,ts,jsx,tsx,mdx}',
+    './components/**/*.{js,ts,jsx,tsx,mdx}',
+  ],
+  theme: {
+    extend: {},
+  },
+  plugins: [],
+}
+
+export default config