fix: resolve all Week 2 performance & security issues (F1.10–F1.19)

Security:
- Replace maybe_serialize() in cookies with json_encode() (PHP object injection fix)
- Add PayPal webhook signature verification
- Add current_user_can('manage_options') to all 18 admin-ajax handlers

Performance:
- Remove flush_rewrite_rules() from init hooks (Thankyou + Payment)
- Add activation/deactivation hooks for flush_rewrite_rules
- Cache currency, country, flags JSON reads in static variables
- Add server-side pagination to Customer::formipay_tabledata_customers()
- Optimize Order::formipay_tabledata_orders() with COUNT(*) GROUP BY

Cleanup:
- Delete Paypal.phpbak backup file
- Fix timezone hardcode Asia/Jakarta → wp_timezone_string()
- Create uninstall.php for proper cleanup on uninstall

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

includes/Render.php

@@ -68,7 +68,7 @@ class Render {
         }
         ob_start();
 
-        $timezone = get_option('timezone_string', 'Asia/Jakarta');
+        $timezone = wp_timezone_string();
 
         $require_login = formipay_get_post_meta($post_id, 'require_login') === 'on';
         $scheduled = formipay_get_post_meta($post_id, 'schedule_toggle') === 'on';