Add optional Show on All Desktops mode

[zhouzhou-1921]

Summary

I'd like to propose an optional Show on All Desktops mode for lil agents.

Current behavior

From both the current implementation and the public docs, the app seems to be built around the active Space plus display-specific visibility. That makes sense for the current behavior, and I don't think the default needs to change.

Proposal

Add an opt-in setting that keeps the character windows visible across macOS Spaces, while leaving the current behavior as the default.

Technical notes

• The current window setup uses .moveToActiveSpace for the main character window, the popover window, and the thinking bubble.
• The app already has dock-aware / screen-aware visibility logic and display pinning.
• The README and release notes mention Dock behavior and multi-display fixes, but I didn't see an all-Spaces mode described there.

Compatibility considerations

The main thing I'd want to preserve is the existing Display behavior:

• users who do nothing should get the current behavior
• display pinning should still work
• the all-desktops option should be app-wide, not a per-window setting

Questions

• Would you be open to this as a small opt-in feature?
• If so, does it make sense to keep the toggle inside the existing Display submenu?