[codex] refactor notification legacy structure

[zombieJ]

Summary

• Move the notification implementation to the new Notification / NotificationList structure
• Remove the unused src/legacy implementation after replacing the remaining context dependency
• Keep tests aligned with the original coverage, adjusting only for description and DOM changes, plus a minimal custom progress component test

Validation

• npm test
• npm run lint
• npm run compile

[socket-security]

Warning

Review the following alerts detected in dependencies.

According to your organization's Security Policy, it is recommended to resolve "Warn" alerts. Learn more about Socket for GitHub.

Action	Severity	Alert  (click "▶" to expand/collapse)
Warn		Critical CVE: tschaub npm gh-pages vulnerable to prototype pollution CVE: GHSA-8mmm-9v2q-x3f9 tschaub gh-pages vulnerable to prototype pollution (CRITICAL) Affected versions: < 5.0.0 Patched version: 5.0.0 From: package.json → npm/gh-pages@3.2.3 ℹ Read more on: This package | This alert | What is a critical CVE? Next steps: Take a moment to review the security alert above. Review the linked package source code to understand the potential risk. Ensure the package is not malicious before proceeding. If you're unsure how to proceed, reach out to your security team or ask the Socket team for help at support@socket.dev. Suggestion: Remove or replace dependencies that include known critical CVEs. Consumers can use dependency overrides or npm audit fix --force to remove vulnerable dependencies. Mark the package as acceptable risk. To ignore this alert only in this pull request, reply with the comment @SocketSecurity ignore npm/gh-pages@3.2.3. You can also ignore all packages with @SocketSecurity ignore-all. To ignore an alert for all future pull requests, use Socket's Dashboard to change the triage state of this alert.

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