Initiative: Multi-tab scan state consistency

[techmore]

Goal\nEnsure all open tabs reflect accurate scan/report state, progress, and ownership.\n\n## Scope\n- Broadcast active job status to existing tabs\n- Ensure non-owner tabs receive live scan updates\n- Resolve idle/active state discrepancies when jobs are running\n\n## Related Issues\n- #127 Existing open tabs do not receive live scan updates when another tab starts a scan\n- #128 Existing tabs receive idle job_status instead of owner job state during active multi-tab scans\n- #140 Bug: Second tab does not mirror active scan state and in-progress indicators