Follow-Up to #14: Add availability-gated Liquid Glass

[alvst]

Summary

Adds the macOS 26 Liquid Glass presentation while retaining the current readable fallback everywhere else.

• macOS 26 and later wrap the existing hosted BarView in NSGlassEffectView.
• The dark panel tint remains to protect white text and card chrome readability.
• Earlier supported macOS versions retain the existing VisualEffectView plus tint layering unchanged.
• The nonactivating panel, card layout, resize behavior, and motion are unchanged.

Dependency

This draft is based on #55 and should merge after it. It will be rebased onto main once its dependencies land.

Validation

• swift build (macOS 14 deployment target; SDK 26.5)
• git diff --check

Screenshot

The UI automation runtime could not attach to the unbundled demo executable, so no capture is attached. Any later visual QA will use Debug Demo only (Pesty --demo).