Follow-Up to #33: Add a right-click delete action for Paste Stack decks

[alvst]

Summary

Adds a destructive right-click action to saved Paste Stack deck cards.

• Right-clicking a deck now shows Delete Paste Stack with the native trash symbol.
• The action removes only that saved deck; the underlying clipboard history remains intact.
• Deleting the active deck cleanly switches to the next saved stack when one exists.

Dependency

Depends on #33 (saved Paste Stack deck support). This branch is based directly on that feature so it does not pull in unrelated preview changes.

Validation

• swift build
• Debug Demo build (Pesty --demo)

Screenshot

Debug Demo; Pesty window only.