fix: resolve install.sh mount point before copying Spool.app

[chumyin]

Summary

• remove -quiet from hdiutil attach so the installer can parse the mounted volume path
• fail early if the mount point cannot be resolved or if the mounted DMG does not contain Spool.app
• keep the detach step quiet without depending on attach output suppression

Validation

• ran bash -n packages/landing/public/install.sh
• ran the patched installer with a temporary install directory and verified it copied Spool.app
• verified the installed bundle reports com.linkclaw.spool and version 0.3.1

Closes #35

[doodlewind]

Thanks for the fix!