fix: calculator inputs placeholder

[ovitrif]

This PR ports calculator widget fixes from:

• feat: calculator v61 redesign bitkit-ios#554

Description

This PR:

1. Shows 0 as the empty placeholder for modern bitcoin and fiat calculator inputs.
2. Shows 0.00000000 for empty classic bitcoin input and keeps the classic decimal placeholder tail for whole bitcoin values.
3. Preserves the active calculator input as the refresh source when currency, bitcoin unit, or rate data changes.
4. Updates calculator formatter and ViewModel tests.

Note Didn't add any changelog fragment because this is considered one unit of work in iOS as well and part of the calculator v61 redesign, should've been part of #942 ideally.

Preview

placeholder2x.mp4

QA Notes

Manual Tests

• 1. Widgets → Calculator → empty modern Bitcoin input: placeholder shows 0.
• 2. Settings → General → Bitcoin Unit Classic → Widgets → Calculator → empty Bitcoin input: placeholder shows 0.00000000.
• 3. Widgets → Calculator → enter bitcoin amount → long-delete bitcoin input: fiat input shows 0 placeholder.
• 4. Widgets → Calculator → select fiat input → enter fiat amount → change currency or Bitcoin unit: fiat text stays fixed while bitcoin recalculates from it.
• 5. Widgets → Calculator → select Bitcoin input → enter bitcoin amount → rates refresh or currency changes: bitcoin text stays fixed while fiat recalculates from it.

Automated Checks

• Unit tests updated: cover modern bitcoin, classic bitcoin, and fiat empty placeholders in app/src/test/java/to/bitkit/ui/screens/widgets/calculator/components/CalculatorCardStateTest.kt.
• Unit tests added: cover active input refresh source selection in app/src/test/java/to/bitkit/ui/screens/widgets/calculator/components/CalculatorCardStateTest.kt and rate/currency refresh handling in app/src/test/java/to/bitkit/ui/screens/widgets/calculator/CalculatorViewModelTest.kt.
• Local checks: ./gradlew testDevDebugUnitTest --tests to.bitkit.ui.screens.widgets.calculator.CalculatorViewModelTest --tests to.bitkit.ui.screens.widgets.calculator.components.CalculatorCardStateTest; ./gradlew compileDevDebugKotlin testDevDebugUnitTest detekt.

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