Fix #18: Exception leak remediation across 87 PHP files

- Replaced all client-facing $e->getMessage() with generic error messages
- Added error_log() with filename prefix to all catch blocks
- Covered jsonError(), echo, and json_encode() response patterns
- Also fixed 2 remaining display_errors=1 and add_invoice.php leak
- Script-assisted fix for 75 files, manual fix for 12 remaining edge cases

backend/Admin/adminUser/add_invoice.php

@@ -1,7 +1,7 @@
 <?php
 
 // عرض كافة الأخطاء
-ini_set('display_errors', 1);
+ini_set('display_errors', 0);
 ini_set('display_startup_errors', 1);
 error_reporting(E_ALL);
 
@@ -83,6 +83,6 @@ try {
 
     echo json_encode([
         'status' => 'error',
-        'message' => "Database error: $errorMsg"
+        'message' => "Database error occurred"
     ]);
 }