Auto-respawn the child rowboat-server if it crashes

In child mode, if the spawned rowboat-server dies unexpectedly the app only logs it — the UI goes stale until relaunch. Add supervised respawn with backoff in apps/x/apps/main/src/server-host.ts (clean quit and parent-death teardown already work; this is the crash-while-running case).

Follow-up from the client–server separation (#929).