Refresh pending dev upgrades

[fazxes]

Summary

• Check for new dev builds every minute while an installed update awaits Ctrl+G.
• Replace superseded pending builds without restarting the active conversation or downloading unchanged builds again.
• Discard a downloaded dev build if the published target changes before installation.
• Prevent Ctrl+G from racing replacement and cancel stalled manifest checks during reload or shutdown.
• Keep a ready update available when discovery fails, and retry failed replacements without advertising the superseded build as ready.
• Restore the ready prompt without reinstalling when the published target returns to the installed build.