On macOS the expanded player window built ExpandedPlayerSheet directly in
a fresh NSHostingController on every open. AppKit doesn't composite the
window until that heavy view finishes its first layout pass, so the click
felt like it waited for data to load before anything appeared.
Host a lightweight two-phase root (black + spinner) that composites
immediately, then defer building ExpandedPlayerSheet by one runloop so the
window shows at once and fades in with visible loading — matching iOS.
Because assigning contentViewController resizes the window to the hosting
controller's fitting size (tiny for the placeholder), force the content
size back to the seeded 16:9 initialSize so the window opens full-sized
during the loading phase too.
