Added full CRUD operations for line items in draft invoices to enable recording of existing due amounts during student enrolment. This feature is perfect for business onboarding scenarios where students have outstanding dues from previous periods.

✨ Key Features

• ➕ Create new line items with custom descriptions and amounts

• ✏️ Edit existing line items (description, quantity, unit price, discount)

• 🗑️ Delete line items with confirmation dialog

• 🔄 Automatic invoice recalculation when line items are modified

• 🔒 Draft-only editing - all operations restricted to draft invoices (status 0)

🎨 Enhanced User Interface

• 📋 Invoice preview enhancements with "Add Item" button for draft invoices

• ⚙️ Edit/delete actions for each line item in draft invoices

• ⚡ Real-time amount calculation in forms

• 📱 Responsive design with Dialog (desktop) and Drawer (mobile) modals

💼 Business Impact

Enables complete financial tracking from day one by allowing users to:

• 📊 Add manual line items like "Outstanding dues from December 2024" with exact amounts

• 🎯 Record pre-enrollment amounts during student onboarding

• 📈 Maintain complete financial visibility and reporting accuracy

🔧 Technical Improvements

• ✅ Comprehensive validation schemas for all line item operations

• 🛡️ Enhanced TypeScript type safety across invoice management

• 🚀 Improved form submission handling with auto-close functionality

• ⚠️ Proper error handling and user feedback throughout