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>

package.json

@@ -81,9 +81,11 @@
     "@playwright/test": "^1.52.0",
     "@swc-node/register": "1.10.9",
     "@swc/cli": "0.6.0",
+    "@tailwindcss/postcss": "^4.1.17",
     "@types/node": "^22.5.4",
     "@types/react": "19.1.8",
     "@types/react-dom": "19.1.6",
+    "autoprefixer": "^10.4.21",
     "copyfiles": "2.4.1",
     "cross-env": "^7.0.3",
     "eslint": "^9.23.0",
@@ -93,6 +95,7 @@
     "next": "15.4.4",
     "open": "^10.1.0",
     "payload": "3.37.0",
+    "postcss": "^8.5.6",
     "prettier": "^3.4.2",
     "qs-esm": "7.0.2",
     "react": "19.1.0",
@@ -101,6 +104,7 @@
     "sharp": "0.34.2",
     "sort-package-json": "^2.10.0",
     "stripe": "^18.5.0",
+    "tailwindcss": "^4.1.17",
     "typescript": "5.7.3",
     "vite-tsconfig-paths": "^5.1.4",
     "vitest": "^3.1.2"