Add native fullscreen for macOS sheet-mode player via in-window overlay

The fullscreen button did nothing in sheet mode: AppKit ignores
toggleFullScreen on a window attached as a sheet, and the old fallback
targeted the key window — the sheet itself. Two other strategies were
ruled out first: fullscreening the sheet's parent is silently refused
while any sheet is attached, and resizing the sheet's backing window to
screen size NaN-asserts inside AppKit's sheet positioning even without
animation.

Working approach: swap the sheet for a plain overlay in the main window
and native-fullscreen the now sheet-free main window.

- isMacPlayerFullScreenOverlay on NavigationCoordinator dismisses the
  sheet (presentation binding excludes it; the setter ignores the
  dismissal write-back) and mounts ExpandedPlayerSheet as a black-backed
  overlay in ContentView
- ExpandedPlayerWindowManager fullscreens the parent after the sheet
  detaches, verifies AppKit accepted it (rolls back if refused), hides
  the window toolbar (NSWindow keeps it as a strip above the overlay
  otherwise) and restores it on exit
- a didExitFullScreen observer restores the sheet on any exit path
  (button, Esc, green button, Window menu); closing the player while
  fullscreen exits the main window's fullscreen too
- scheduleSharedViewAdoptionRetry on entry re-homes the shared render
  view once the dismissing sheet's window loses visibility — its steal
  is declined mid-dismissal and nothing else retried, leaving the
  overlay black until the next video load

Yattee/Services/Navigation/NavigationCoordinator.swift

@@ -115,6 +115,11 @@ final class NavigationCoordinator {
     /// Whether the player is currently expanding (for animation coordination with mini player).
     var isPlayerExpanding = false
 
+    /// macOS only: true while the inline player runs in fullscreen overlay
+    /// mode — sheet dismissed, player shown as an overlay in the main window,
+    /// main window native-fullscreen. Set/cleared by ExpandedPlayerWindowManager.
+    var isMacPlayerFullScreenOverlay = false
+
     /// Whether the player is currently collapsing (for animation coordination with mini player).
     var isPlayerCollapsing = false