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>

admin/functions.php

@@ -26,11 +26,13 @@ function formipay_field_type_collection() {
 }
 
 function formipay_currency_array() {
-
+    static $cache = null;
+    if ($cache !== null) {
+        return $cache;
+    }
     $json = file_get_contents(FORMIPAY_PATH . 'admin/assets/json/currencies.json');
-    $array = json_decode($json, true);
-    return $array;
-
+    $cache = json_decode($json, true);
+    return $cache;
 }
 
 function formipay_is_multi_currency_active() {
@@ -146,22 +148,26 @@ function get_global_currency_array() {
 }
 
 function formipay_country_array() {
-
+    static $cache = null;
+    if ($cache !== null) {
+        return $cache;
+    }
     $json = file_get_contents(FORMIPAY_PATH . 'admin/assets/json/country.json');
-    $array = json_decode($json, true);
-    return $array;
-
+    $cache = json_decode($json, true);
+    return $cache;
 }
 
 function formipay_get_flag_by_currency($currency) {
-
+    static $flags = null;
     if(strpos($currency, ':::')){
         $currency = explode(':::', $currency);
         $currency = $currency[0];
     }
-    $json = file_get_contents(FORMIPAY_PATH . 'admin/assets/json/flags.json');
-    $array = json_decode($json, true);
-    foreach($array as $country){
+    if ($flags === null) {
+        $json = file_get_contents(FORMIPAY_PATH . 'admin/assets/json/flags.json');
+        $flags = json_decode($json, true);
+    }
+    foreach($flags as $country){
         if($currency == $country['code']){
             return $country['flag'];
         }