Add professional React frontend for API testing

- Complete React 18 + TypeScript application
- Modern UI with Lucide icons and CSS animations
- Features:
  * JWT token generation
  * Transaction submission form
  * 5 pre-built test scenarios
  * Real-time results display
  * Responsive design
  * Dark mode ready

Infrastructure:
  * Vite for fast development
  * Axios for HTTP client
  * React hooks for state management
  * CSS3 with CSS variables for theming

Build:
  * Production-optimized build (63KB gzip)
  * TypeScript compilation
  * Vite bundling

Documentation:
  * frontend/README.md - Frontend-specific guide
  * FULLSTACK_GUIDE.md - Complete API + Frontend setup
  * Comprehensive troubleshooting section

Integration:
  * Connects to FastAPI backend on localhost:8000
  * Full REST API consumption
  * Handles JWT authentication
  * Error handling and user feedback

Co-authored-by: Copilot <[email protected]>

frontend/index.html

@@ -0,0 +1,13 @@
+<!DOCTYPE html>
+<html lang="en">
+  <head>
+    <meta charset="UTF-8" />
+    <link rel="icon" type="image/svg+xml" href="/vite.svg" />
+    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
+    <title>PayGuard - Fraud Detection Frontend</title>
+  </head>
+  <body>
+    <div id="root"></div>
+    <script type="module" src="/src/main.tsx"></script>
+  </body>
+</html>