feat: add optional Day-of-Week Activity Excel sheet

[davidamunga]

Summary

• Adds an optional Day-of-Week Activity Excel analysis sheet (Mon–Sun breakdown, weekday vs weekend, key insights)
• Wires the toggle into export options, XLSX export, screenshot harness, README, and a one-line changeset

Test plan

• Select Excel export → enable Day of Week under Additional Sheets → Analytics
• Export a statement and confirm the Day-of-Week Activity workbook tab appears
• Verify peak day highlighting and weekday vs weekend totals look sensible
• Confirm Select all / Deselect all includes the new toggle