Cherry-pick of PR #2796 by @ai-ag2026, squashed from 5 author commits onto current master:
- dcee0563 fix: drop stale optimistic sidebar rows
- 3a73400d fix: clear stale busy state before send
- 46c3b902 fix: preserve server idle rows during optimistic merge
- de51d271 fix: let chat start survive pre-start UI errors
- d2f5c906 fix: hide nonfatal pre-start send warnings
Authorship preserved via --author. Code-only squash (no CHANGELOG).