In sheet (inline) mode the expanded player kept a fixed size and never
reacted to the video's aspect ratio, unlike window/floating mode. Drive
the sheet's backing NSWindow size from the video aspect ratio, reusing
the same sizing math as the standalone window.
- Extract shared sizing into ExpandedPlayerWindowManager.fittedPlayerSize
and add fittedSheetSize for the sheet path.
- Add SheetWindowResizer (NSViewRepresentable) + .sheetWindowSize modifier
that resizes the hosting sheet window, since .presentationSizing(.fitted)
only fits once at presentation.
- Let the sheet content fill the window and paint the window background
black so animated resizes stay gap-free with no white bars.
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.
