Fix Linux editor browser lifetime

[neenaoffline]

Summary

• launch the Linux editor in Chrome app mode when available, with Firefox as a fallback
• avoid WebUI bridge failures for the independently served local page
• keep the local editor service alive until SIGINT or SIGTERM
• permit the packaged command to launch supported browsers

Validation

• Deno type checks passed
• window test passed
• package build passed
• runtime remained alive beyond the previous 30-second timeout and exited cleanly on SIGTERM