Sprint 6E: live explain-why trace UI

[samrusani]

Summary

• replace the fixture-only /traces route with live trace list, detail, and event reads when API configuration is present
• preserve explicit fixture fallback and bounded unavailable or partial-detail states when live data is absent or incomplete
• keep the sprint inside the existing shipped trace review backend contract with no broader scope expansion

Verification

• npm run lint in apps/web: PASS
• npm test in apps/web: PASS (3 files, 13 tests)
• npm run build in apps/web: PASS

Notes

• /traces now uses the shipped trace review APIs when configured and stays explicit about fallback behavior when not configured
• no backend, Gmail, Calendar, auth, runner, or broader scope expansion entered the sprint