Integrate the experimental Android screenshot testing plugin into the `core:ui` module. This includes initial visual regression tests for Alert components and updates to the CI workflow for automated validation.
- core/ui: Add `AlertScreenshotTest` and screenshot test dependencies
- gradle.properties: Enable `android.experimental.enableScreenshotTest`
- libs.versions.toml: Add `com.android.compose.screenshot` plugin and validation API
- reusable-check.yml: Add screenshot validation tasks to GitHub Actions workflow
