fix: calculation of P/L in balance sheet

[barredterra]

General Ledger

Rows	Description
2, 3	25,000.00 received via bank transfer (equity)
4, 5, 6	119.00 received via bank transfer (100.00 revenue, 19.00 VAT)
7, 8	19.00 paid via bank transfer (advance payment of VAT, offsetting the VAT liability)

Balance Sheet (Before)

Balance Sheet (After)

Important

I'm not sure why [-2] was used in the first place, if this was ever correct and, if so, what broke it. Some deeper analysis might still be needed.

[barredterra]

The added test case is passing:

Unrelated unit test failing (fails on many unrelated PRs)