Upload System Modernization

[magicoli]

Replace hardcoded jQuery File Upload with composer package

Tasks

• Replace current custom upload library with blueimp/jquery-file-upload composer package
• Create webui/upload_handler.php - Modern upload handling (replaces per-client implementations)
• Create engine/api/upload.php - API endpoint for uploads with authentication
• Implement proper progress tracking and error handling
• Add support for drag & drop and multiple file selection
• Support upload on any page with proper user authentication
• Integrate with WordPress authentication via API (existing wrap-wp plugin)
• Optional authentication: no login required for viewing, login required for upload
• Deploy independently - removes per-client jQuery File Upload copies, fixes upload issues