refactor: make FiatValue currency explicit

[qorexdevs]

covers the FiatValue constructor cleanup from #66:

• adds FiatValue.zero(currency) for explicit zero values
• removes default amount/currency from FiatValue.__init__
• updates PLN accumulators to use explicit Currency.ZLOTY

all 190 tests pass locally

ref #66

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ All tests successful. No failed tests found.

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