fix(frontend): render session.error events in managed session chat (#2598)

## Summary

- `ChatPanel.tsx`: add a `session.error` branch to both
`eventsToMessages` (initial load) and `handleManagedEvent` (live SSE) so
failed turns surface as an error bubble in the chat; clear the in-flight
reply's pending flag on failure
- `MessageBlock.tsx`: add an `error` message role with a red-tinted
style
- Backend already persists `session.error` (payload `{error: {type,
code, message}}`) via `SessionTurnRunner.failTurn` — this PR only makes
the console render it

Fixes #2596