Fix realized capital gains calculation and add period adjustment

[Morpheus1w3]

This change improves how realized capital gains are displayed in the performance calculation.
The existing period-based realized gains remain unchanged.

A new purchase-based realized gains line is added. It uses the actual historical purchase cost, based on FIFO or moving average. This helps users compare sales in the selected period against the original purchase cost.

A new adjustment line is added to keep the overall period-based performance calculation consistent.
The total portfolio value and the existing performance logic are not changed.

####Review note
The purchase-based value is built from transactions before the reporting period, so sales inside the selected period can be compared against the historical purchase cost.

The adjustment line keeps the existing period-based performance calculation consistent.

[Morpheus1w3]

At name.abuchen.portfolio.ui/src/name/abuchen/portfolio/ui/views/dashboard/PerformanceCalculationWidget.java a minor Bugfix Was implemented.

For example "Cash Fremdwährungsgewinne" is basically an addition. However, it could happen that the amount is negative. So it' lead into confusion, if the calculation method is an addition, but the amount is negative and subtracted.

[Sn1kk3r5]

closes: #5662

[prieurcath]

Thanks for this :)

[mierin12]

Hmm if this goes through we need to be extra extra clear about what are those two new values. What is the meaning of the 336.8 EUR value ?
There are two different definitions for realized/unrealized gains now. Realized gains are now related to purchase cost, but unrealized gains remains related to the start of the period ? Why not related to the purchase cost too then ?

My understanding is that it was simply not the task of Performance/Calculation to provide such kind of values.
Performance Calculation was meant to provide information about the performance since a start date. Whatever happened before is irrelevant, as at the start everything is considered invested, both gains and principal.
https://forum.portfolio-performance.info/t/realized-capital-gains-doesnt-match-closed-trades/38737
#3387

Maybe Trades view or Payment/Closed Trades view or the Trades Profit/Loss widget are more usable for this kind of topic. But each of them have different characteristics but we can work on them:

View	Account filter ?	Period filter ?	FIFO and moving average ?	Before or after taxes and fees ?
Trades	No	No	Both	Both
Payment/Closed trades	Yes	Yes	FIFO only	Before only
Trades Profit/Loss widget	Yes	Yes	Both	After only

(before tax and fees = without tax and fees = pure price gains)
(after tax and fees = with tax and fees = gains reduced by tax and fees)

Another thing, I am assuming those "taxable gains" determination should be gains AFTER fees, right ? But the Performance Calculation view only provides Gains that are before tax and fees. So if you calculate your taxable gains from this view, then it means you consider that you have to pay tax on pure price gain, not yet reduced by the differents fees. So you consider you have to pay taxes on your fees..

All in all, PP is not made for tax calculation.

[Morpheus1w3]

Existing calculation method is even misleading. Alternative proposal as below.

Calculation Method (Performance)

INITIAL
+ CAPITAL_GAINS
+ EARNINGS
- FEE
- TAX
+ FOREX
= FINAL

[prieurcath]

Even "Portfolio Performance is focusing the performance of investments, t" . the calculation on performance when Securities have been sold the profit must be calculated on initial price paid and not on the last cotation. If fact it's the case on curent year just apprear to be different if the acquisition was made the previous year.