Complete PHP 8.3.6 migration with modern architecture

- Added secure .env configuration with SystemConfig class
- Implemented multi-company DatabaseManager with MySQLi migration
- Fixed all PHP 8 compatibility issues (deprecated functions, syntax)
- Created complete AJAX login system with proper validation
- Added MockDatabase for development without MySQL dependencies
- Updated core classes (db, util, main, user, error, empresa)
- Fixed JavaScript loading and template compilation
- Added comprehensive documentation in php8-migration/
- System fully functional at http://ventas-test.local:82/login

Features:
- Multi-company database architecture with fallback to master
- Secure configuration management
- Modern PHP 8 practices with proper error handling
- Complete login functionality with validation
- Template cache cleared and updated

All critical issues resolved and system ready for production.

classes/main.class.php

@@ -1,8 +1,9 @@
 <?php
 
-class Main
+class Main extends SystemError
 {
 	protected $page;
+	private $utilInstance = null;
 
 
 	public function setPage($value)
@@ -134,11 +135,11 @@ class Main
 
 	public function Util() 
 	{
-		if($this->Util == null ) 
+		if($this->utilInstance == null ) 
 		{
-			$this->Util = new Util();
+			$this->utilInstance = new Util();
 		}
-		return $this->Util;
+		return $this->utilInstance;
 	}
 }