FSFPAY Checkout Shortcode WordPress Plugin

Usage:
1. Install plugin by uploading the ZIP in WordPress admin > Plugins > Add New > Upload Plugin.
2. Activate plugin.
3. Go to Settings > FSFPAY and configure API Key, API Secret and endpoints.
4. Add shortcode to any post/page:
   [fsfpay_button amount="9.90" currency="USD" description="Donation" label="Pay with FSFPAY" order_id="12345"]
5. Set webhook URL in FSFPAY panel to: https://your-site.com/wp-json/fsfpay/v1/ipn

Notes:
- The plugin creates the checkout session server-side via an AJAX call to avoid exposing the secret key.
- Webhook handler stores simple logs in WordPress option 'fsfpay_wp_logs'. You can extend to create custom post types or integrate with WooCommerce if needed.
