fix(macOS): show the pet across all Spaces

[CatStack-pixe]

Upstream source

• BongoCat PR #968 by @shuifa
• BongoCat issue #966 by @shuifa

Port assessment

Adapt the macOS NSPanel/window-level change to MochiPaw's custom window plugin.

Acceptance criteria

• The pet remains visible when switching macOS Spaces.
• Preference-window behavior and Dock visibility remain unchanged.
• Verify on a multi-Space macOS installation.