Optimistically render web chat sends

[edwin-zvs]

Summary

• render chat composer sends immediately as optimistic user rows
• reconcile the optimistic row when the canonical transcript event arrives, avoiding duplicate user messages
• remove the optimistic row and restore composer text if the send RPC fails

Testing

• git diff --check
• cargo build
• cargo test -p agentd-e2e --test web_smoke web_client_loads_and_websocket_connects

Notes

• Hot reload is pointed at this worktree's web assets for manual verification.