- Added `invoice_file` attribute to store the filename where the invoice number is persisted.
- Implemented `load_invoice_number` to read the last invoice number from the file on app startup. If the file doesn't exist or contains an invalid number, it defaults to 1.
- Implemented `save_invoice_number` to persist the incremented invoice number to the file after generating a PDF.
- Modified `export_to_pdf` to increment and save the invoice number after successful PDF generation.
