Mutex was Local\ (per Windows session), so a second RDP/fast-user-switching
session bypassed the single-instance guard. A Kestrel bind failure on top of
that had no try/catch, so a second worker (or any process on 47821/47822)
died as an unhandled crash with nothing in the log to explain why the UI
never connects. Mutex is now Global\, bind failures and any last-resort
unhandled exception log a Fatal line and exit(1) instead of crashing raw.
