Follow-up to #31: Persist saved Paste Stacks

[alvst]

What

• Persists saved Paste Stack decks across launches and syncs their snapshots through iCloud Drive.
• Preserves image-file entries while clipboard history is pruned.
• Adds an optional setting to remove saved stacks together with clipboard history.

Dependency

This is a draft follow-up to #31, which is itself the final action layer over #30, #29, and #9. It also reuses the history-retention behavior from #21.

GitHub cannot make an upstream PR target a branch in this fork, so the current diff contains those prerequisite changes. After #9, #21, #29, #30, and #31 merge, this branch will be rebased onto upstream main; the PR will reduce to the persistence-only work.

Validation

• swift build
• swift build -Xswiftc -DMAS
• Tested on Apple Silicon.
• Intel Mac testing was not possible because no Intel Mac is available.

Screenshots

Demo mode; Pesty window only.