fix(app): clear stale timeline errors

[Brendonovich]

Summary

• derive a turn error from its latest assistant message
• remove stale failure rows when the same turn resumes streaming
• add regression coverage for failed-then-recovered turns

Testing

• bun test --preload ./happydom.ts ./src/pages/session/timeline
• bun turbo typecheck
• production timeline streaming benchmark before and after (no regression)