feat(ui): allow customizing Alert close control

[wyf027]

Summary

• Add closeIcon and closeAriaLabel to Alert so dismiss controls can be localized or customized while keeping an accessible name.
• Pass the close button click event to onClose, matching mature Alert APIs that expose close callbacks with event context.
• Cover custom close controls in tests and update the VitePress example to use Tailwind layout classes.

Validation

• GitHub Actions CI run 28903491031 passed for commit b9b07e81ebf5f360e9af6abdef317ab27c78880b.
• CI quality job passed: Install, Lint UI, Typecheck UI, Test UI, Build UI, Build docs.
• Local npm run lint:ui, npm run typecheck:ui, npm run test:ui, and npm run build could not be executed because the local shell runner failed to create a process in this Codex environment.