Follow-Up to #43: Make Paste Stacks optional

[alvst]

Summary

Makes Paste Stacks an optional workflow instead of a permanently visible Paste Bar section.

• Adds a Settings toggle for Paste Stacks.
• When disabled, stops stack capture, removes the tab/start control and deck cards, and returns stack clips to Clipboard history.
• Unregisters the Paste Stack sequence shortcut and dismisses an active collector or Quick Look surface.
• Retains saved stacks, so re-enabling the feature restores them.
• Skips the disabled Paste Stack section during ⌘←/⌘→ navigation.

Follow-up

Follow-up to #43, Move history clips into the Paste Stack. It preserves the existing Paste Stack behavior when enabled and makes the workflow opt-in.

Completion update

• Adds a destructive right-click action to delete a saved deck safely.
• Promotes an adjacent viable deck when the active deck is deleted.
• Uses a compact deck presentation in short Paste Bars.
• Adds a Swift test target with seven state and persistence tests for Paste Stacks.

Dependency

This draft builds on #54’s section-navigation work, so merge #49 and #54 first. It is independent of #51 through #53.

Validation

• swift test — 7 tests passed
• git diff --check

Screenshot

Any visual QA for this PR uses Debug Demo only (Pesty --demo).