fix(ui): tune Notification live region semantics

[wyf027]

Summary

• default non-error notifications to role="status" with polite live-region semantics
• keep error notifications assertive via role="alert"
• allow callers to override urgency through role and aria-live
• update Notification tests and Tailwind docs for the live-region behavior

Validation

• GitHub Actions CI / quality passed for head 48cb681426b087975a3360675638339722af99bb in run 28889749589.
• Passed steps: npm run lint:ui, npm run typecheck:ui, npm run test:ui, npm run build:ui, npm run build:docs.
• Test run included Notification coverage for default polite status regions, error alert regions, explicit urgency overrides, close behavior, controlled mode, and auto-close timing.
• Not run locally: the Codex shell executor is currently failing with Failed to create unified exec process: No such file or directory.

Notes

• codebase-memory-mcp graph tools were not exposed in this session, so this run used GitHub file reads as the AGENTS.md fallback.
• External references: MDN notes role="alert" is assertive and should be used sparingly for important time-sensitive information; Ant Design has a related issue documenting that toast-style notifications should prefer status/polite semantics for non-critical content.