On reconnect to an active agent run the server now replays all events
emitted since the turn started, then switches to live forwarding.
This eliminates the gap where tool cards, thinking blocks and stream
deltas were permanently lost after a network blip.

Server (_AgentRun):
- events: list[dict] buffers every serialised agent event
- broadcast() serialises and appends before putting in subscriber queues
- reconnect flow: subscribe → replay_count snapshot → stream_start →
  replay events[0:replay_count] → live _stream_to_client

Client:
- onStreamStart() removes the frozen ghost message instead of marking
  done=true, so replay cleanly rebuilds the message from scratch
- replayMode flag suppresses animations during replay
- onReplayStart/onReplayEnd handlers set/clear the flag and restore
  animate on the message once live events resume

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>