fix(web): avoid deprecated shadow props

[huytdps13400]

Summary

• use a web-only boxShadow for BaseToast instead of the deprecated React Native shadow* style props
• keep the existing native shadowOffset, shadowOpacity, and shadowRadius behavior unchanged
• keep Android elevation shared across platforms
• add platform-isolated regression tests for both style branches

Because React Native Web preprocesses StyleSheet.create eagerly, this prevents the warning even when an app imports the package with a fully custom toast config and never renders BaseToast.

Fixes #600

Test Plan

• RED: yarn test src/components/__tests__/BaseToast.styles.test.ts --runInBand failed because the web style had no boxShadow and still contained all three deprecated shadow props
• focused regression: 2 tests passed
• yarn quality: type-check, lint, 19 suites / 85 tests passed, 100% statement/branch/function/line coverage
• yarn build
• git diff --check

I also verified the mapping against React Native Web 0.21.2's official StyleSheet preprocessor: any shadow* key triggers the warning and is translated to an equivalent CSS boxShadow string.