Follow-up to #30: Paste stacks into active apps and save them

[alvst]

What

• Pastes a Paste Stack into the app that was active when the action began.
• Adds a Save as Pinboard action for the current stack.

Dependency

This is a draft follow-up to #30, which itself builds on #9 and #29. The actions use the deck APIs introduced by #30, so they are intentionally kept together rather than duplicating that UI.

GitHub cannot make an upstream PR target a branch in this fork, so the current diff includes the parent work. After #9, #29, and #30 merge, this branch will be rebased onto upstream main; the PR will then contain only this actions commit.

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. The target-app paste behavior is not shown because captures are deliberately limited to Pesty.