feat: display per-row exchange rates on FA(3) invoices

[nawrotadam]

Description

FA(3) invoices issued in a foreign currency can carry a different exchange rate (KursWaluty) per invoice line, but currently the exchange rate is not displayed per line. This PR adds per-line visualization of exchange rates and changes when a single rate common to the whole invoice is displayed.

Implemented behaviour:

• If all lines share one rate, the rate is shown in the invoice header along with the note that it is common to all lines. Per-line exchange rates are not shown.
• If lines have different rates, or some lines have no rate, a per-line exchange rate column is added to the positions table, showing each line rate.
• Rates are compared numerically, so equal values formatted differently (e.g. with trailing zeros) are treated as the same rate.
• For correction invoices with a pair of before and after rows (StanPrzed), the common header is never shown. The per-line exchange rate is shown when the rate was actually corrected (before != after), and no exchange rate is shown when it is unchanged (before and after have the same value).

References

Fixes #140

Testing

Unit tests added in ExchangeRateTest, covering five scenarios:

• all lines share one rate
• different rates
• incomplete rates
• correction invoice with a changed rate
• correction invoice without a rate change

Test invoice examples added to test/resources/ExchangeRateTest.