Animate Pesty bar dismissal when switching apps

[alvst]

Summary

• Hide the Paste Bar whenever another app becomes active.
• Start the slide-down before Pesty loses active status, so Command-Tab renders the dismissal rather than abruptly removing the bar.
• Ignore duplicate dismissal events and prevent a stale animation completion from hiding a newly reopened bar.

Root cause

Command-Tab could trigger both the key-resign and workspace-activation paths. They started overlapping hide animations, with the later app switch often preventing the animation from being composited.

Validation

• swift build

Static screenshots are not applicable to this transition behavior.