Add macOS skin (toggle, branch only — not merged to master)

A second desktop style, switchable via a floating Linux⇄macOS pill at the
bottom-left of the dock (persisted in localStorage; ?os=macos|linux override).

macOS mode (body.macos), built as a skin layer over the same codebase:
- Dark macOS theme: system blue (#0a84ff) accent, SF/system font, frosted
  vibrancy windows, gradient wallpaper, dark mode.
- Traffic-light window controls on the left + centered titles; green = zoom
  (zoom is now a toggle for both skins).
- Top menu bar (apple mark, app name, menus, Control Center, Spotlight, live
  clock) + a working Spotlight overlay (⌘/Ctrl+Space) that opens apps or
  routes a question to the Terminal.
- Free-floating cascade windows (auto-tiling/snap disabled in mac mode) with a
  curated landing (hero + About This Mac).
- Magnifying Dock with rounded-square tiles + hover labels.
- Metaphors reframed as Mac apps: About This Mac (fastfetch), Finder (work),
  Notes (vision/Kernel), Terminal.app, Contacts (connect); dmesg lines hidden.
- Mobile + Linux skin unchanged; mac skin degrades cleanly on mobile.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: captkernel