fix(ui): tune Toast live region semantics

[wyf027]

Summary

• Align Toast live-region semantics with Notification: non-error statuses use role="status"/polite updates, while status="error" defaults to role="alert".
• Allow callers to override Toast role and aria-live when a message needs custom urgency.
• Document the accessibility behavior and update the VitePress example to use Tailwind layout classes.

Validation

• GitHub Actions CI run 28900440301 passed for commit fbb07be660fde61cc7818b75e462b9291b079fa4.
• 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.

[ecc-tools]

ECC bundle files are already tracked in this repository. Skipping generation of another bundle PR.