I need to modernize the GST Invoice UI in my PHP ERP system by integrating a modern 3D responsive design with the existing backend code.

REFERENCE TEMPLATE:
I have a modern HTML template that shows EXACTLY the design and UX I want to achieve. Please analyze the file: Updated_UI_GST_Invoice.html as the target design template.

KEY DESIGN ELEMENTS FROM TEMPLATE:
- Card-based sidebar navigation instead of tabs
- 3D visual effects with glass morphism
- Tailwind CSS for styling
- Responsive grid layouts
- Modern form inputs with 3D hover effects
- Progress indicator in header
- Floating action buttons
- Smooth animations and transitions
- Professional gradient backgrounds

CURRENT CODEBASE TO MODERNIZE:
- InvoiceGST.php (main entry point)
- InvoiceGST.inc (main UI generation - THIS IS THE PRIMARY TARGET)
- Filldropdown.php (AJAX handler)
- validation.js (form validation)
- Related BO (Business Object) files
- CSS and JavaScript dependencies

INTEGRATION REQUIREMENTS:
1. Use the HTML template as the design foundation
2. Replace existing table-based layout with modern card sections
3. Preserve ALL existing PHP form field names and POST variables
4. Maintain ALL JavaScript validation functions
5. Keep ALL AJAX functionality intact
6. Ensure ALL popup windows continue to work
7. Preserve business logic and workflow
8. Add modern UX features from template (progress bars, notifications, etc.)

SPECIFIC SECTIONS TO MODERNIZE (matching template structure):
1. Basic Information (Invoice details, dates, series)
2. Customer & Address (Customer selection, billing/delivery addresses)
3. Product Details (Product table with batch information)
4. Batch Allocation (Batch selection and FEFO logic)
5. Transport & Packing (Transport details, packing information)
6. Other Charges (Tax calculations, charges, summary)

TECHNICAL APPROACH:
1. Analyze existing InvoiceGST.inc structure
2. Map all form fields to modern UI components
3. Preserve PHP echo statements and logic
4. Replace HTML output with Tailwind-based components
5. Enhance JavaScript for modern interactions
6. Test all AJAX endpoints and validation rules

DELIVERABLES:
1. Modernized InvoiceGST.inc with template-based design
2. Updated JavaScript files with enhanced UX
3. Integration documentation
4. Testing checklist for functionality verification
5. Migration notes for any breaking changes

Please start by analyzing both the reference template and existing codebase, then provide a modernization plan.