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/serviceapp/login.php

@@ -127,7 +127,7 @@ try {
     // ✅ FIX M-02: إخفاء تفاصيل الخطأ في الإنتاج
     $debugMode = getenv('APP_DEBUG') === 'true';
     securityLog("[ServiceApp Login Error]", ['msg' => $e->getMessage()]);
-    jsonError($debugMode ? "Server error: " . $e->getMessage() : "Server error. Please try again later.", 500);
+    jsonError("Server error. Please try again later.", 500);
 }
 
 exit();