The primary backend could exit before readiness on every restart (for example
with a corrupt statev2.sqlite) and the user saw no window and no error, because
the renderer is served by that backend.

After three consecutive exits without readiness, the pool now shows a native
error box with the exit reason and the server-child.log path. Restarts keep
going in the background, and the streak resets once the backend becomes ready.

Closes #10517

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>