Add a "Payd taxes" field to transactions

Problem:
In Germany capital gains taxes are withheld directly at the point of sale by the broker (Kapitalertragsteuer, Solidaritätszuschlag). The proceeds credited to the account are therefore already net of taxes.

Currently this cannot be recorded properly. As a result:
The realized return shown in the tracker is systematically overstated, because it reflects the gross gain rather than the amount actually received.

Proposed solution:
Add an optional "Payd taxes" input field in the "Add transaction" dialog, placed directly next to the existing "Fee" field. It should behave analogously to "Fee":

Reduces net proceeds on a sale. Flows into realized return, total return, and cash balance calculations. Is reported separately from fees, so users can see costs vs. taxes distinctly.