Follow Up to #33: Move history clips into the Paste Stack

[alvst]

Dependency

This is a stacked follow-up to #33 and the underlying Paste Stack series (#29–#31). It should be reviewed and merged after that series. The feature commit is 3335d23; prerequisite commits appear because upstream main does not yet contain Paste Stack.

What changed

• Adds Move to Paste Stack to Clipboard History card menus.
• Starts/resumes the active Paste Stack when a historical clip is added.
• Avoids duplicate membership across saved stacks while preserving history and pinboard data.
• Preserves image previews and writes the active stack through the established persistence path.

Validation

• swift build
• git diff --check fork/focused/paste-stack-history-followup..HEAD
• Demo-mode UI verification of a history item being moved into the active Paste Stack.

Screenshot

Demo mode — Pesty window only.