Fix Node.js version compatibility and improve documentation

- Update amplify.yml to use Node.js 20 to resolve engine compatibility warnings
- Add .nvmrc file to ensure consistent Node.js version across environments
- Replace basic README with comprehensive project documentation including features, setup instructions, and deployment options

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

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

amplify.yml

@@ -3,6 +3,9 @@ frontend:
   phases:
     preBuild:
       commands:
+        # Set Node.js version to 20
+        - nvm use 20
+        - node --version
         - npm ci
         - |
           # Retrieve the secret from Amplify's secret store