Document CSV export feature

[projekta2]

`## Description
TabCost Pro can export daily history to CSV (Pro feature). The README mentions it but doesn't explain how to use it.

Tasks

• Add a section "Export to CSV" in README
• Explain steps: open Options → History → click "Export CSV"
• Add a note about file format (UTF-8 with BOM)
• (Optional) Add a screenshot

Good first issue – I can review and merge.`

[projekta2]

Thanks for looking at this issue.

To clarify: the CSV export feature is already implemented and works. This issue is purely about documentation – adding a short section to the README explaining how to use it.

Steps to export a CSV (Pro users):

1. Open TabCost popup → click "Advanced History & Reports"
2. In the control panel, go to the History tab
3. Click the "Export CSV" button

The file contains the daily history table (date, cost) and opens in Excel/Google Sheets without extra formatting.

I can help review the PR when you're ready. If you'd like to take this issue, just leave a comment and I'll assign it to you.