Multi line splits and prepayments closing balance

Briefcase now displays two views of invoice data:

• VAT summary – line items grouped by VAT rate
• All line items – every item listed individually

Users can toggle between these views using the "Show all line items" button. The first view aggregates entries by VAT rate, while the second displays every individual line item for detailed inspection.

Briefcase now shows the running balance of each individual prepayment on the prepayments tracker page. This allows users to identify what comprises their total prepayments balance.

Additionally, prepayment CSV exports now include two tables:

• First table: monthly amount expensed by each prepayment
• Second table: monthly closing balance of each prepayment