test: add calculator device coverage

[ovitrif]

Fixes #937

Description

This PR:

1. Moves calculator widget input/conversion behavior into the calculator view model so the card only renders state and forwards events.
2. Adds unit coverage for calculator hydration, sanitizing, conversion, persistence, and currency refresh behavior.
3. Adds a device UI integration test for the calculator card that types into BTC and fiat inputs, verifies rendered values, and verifies persisted widget state.
4. Splits Android instrumentation tests into annotated lanes for Compose UI, device UI integration, device storage integration, and core service integration.

Preview

N/A

QA Notes

Manual Tests

• 1. Widgets → Calculator Preview → enter BTC amount: fiat field updates and saved widget keeps both values.
• 2. Widgets → Calculator Preview → enter fiat amount: BTC field updates and saved widget keeps both values.

Automated Tests

• Unit tests added: cover calculator view model behavior in CalculatorViewModelTest.kt.
• Android test added: covers calculator card device interaction and persistence in CalculatorCardIntegrationTest.kt.
• Android test lanes added:
• connectedDevDebugComposeAndroidTest
• connectedDevDebugDeviceUiIntegrationAndroidTest
• connectedDevDebugDeviceStorageIntegrationAndroidTest
• connectedDevDebugCoreServiceIntegrationAndroidTest
• connectedDevDebugDeviceIntegrationAndroidTest

Checks

• Ran locally: ./gradlew compileDevDebugKotlin.
• Ran locally: ./gradlew compileDevDebugAndroidTestKotlin.
• Ran locally: ./gradlew testDevDebugUnitTest.
• Ran locally: ./gradlew detekt --rerun-tasks.
• Ran on Pixel 10 Pro:

ANDROID_SERIAL='$ID' ./gradlew connectedDevDebugDeviceUiIntegrationAndroidTest
ANDROID_SERIAL='$ID' ./gradlew connectedDevDebugDeviceStorageIntegrationAndroidTest
ANDROID_SERIAL='$ID' ./gradlew connectedDevDebugCoreServiceIntegrationAndroidTest
ANDROID_SERIAL='$ID' ./gradlew connectedDevDebugDeviceIntegrationAndroidTest