feat: add optional Charges/Fees sheet to Excel exports

[DavidAmunga]

Add Optional Charges/Fees Sheet to Excel Exports

Overview

This PR adds a new optional export feature that creates a separate "Charges & Fees" sheet when exporting M-Pesa statements to Excel format. This enhancement helps users better analyze their transaction fees and charges.

What's New

• New Export Option: Checkbox to include "Charges/Fees Sheet" when Excel format is selected

Service Layer Updates

• ExportService: Updated to pass export options through the service chain
• XlsxService: Enhanced to generate additional sheets based on options
  • New addChargesSheet() method for creating the charges worksheet

Testing Scenarios

• Excel export with charges sheet enabled
• Excel export with charges sheet disabled
• CSV export (no charges option shown)
• Format switching maintains option state
• Empty charges (no sheet created if no charges found)

Screenshots

Add screenshots showing:

1. New checkbox option in the export UI

2. Generated Excel file with both sheets

Benefits for Users

• Better Financial Awareness: Clear visibility into M-Pesa fees and charges

Backward Compatibility

• Existing CSV exports unchanged
• Default Excel exports unchanged (charges sheet is optional)
• No breaking changes to existing APIs