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/ContentView.swift

@@ -57,6 +57,17 @@ struct ContentView: View {
             // Mini player overlay (macOS only)
             #if os(macOS)
             miniPlayerOverlay(appEnvironment: appEnvironment)
+
+            // Fullscreen overlay player: replaces the sheet while the main
+            // window is native-fullscreen (a window with an attached sheet
+            // can't fullscreen, so the sheet is swapped for this overlay).
+            if appEnvironment.navigationCoordinator.isMacPlayerFullScreenOverlay {
+                ZStack {
+                    Color.black
+                    ExpandedPlayerSheet()
+                }
+                .ignoresSafeArea()
+            }
             #endif
         }
         #if os(macOS)
@@ -67,6 +78,9 @@ struct ContentView: View {
         }
         .onChange(of: appEnvironment.navigationCoordinator.isPlayerExpanded) { _, isExpanded in
             if !isExpanded {
+                // Closing the player while in overlay fullscreen also exits
+                // the main window's fullscreen (it was entered for the video).
+                ExpandedPlayerWindowManager.shared.endOverlayFullScreen(exitParentFullScreen: true)
                 ExpandedPlayerWindowManager.shared.hide()
             }
         }
@@ -96,9 +110,17 @@ struct ContentView: View {
         .sheet(isPresented: Binding(
             get: {
                 appEnvironment.navigationCoordinator.isPlayerExpanded &&
-                !appEnvironment.settingsManager.macPlayerSeparateWindow
+                !appEnvironment.settingsManager.macPlayerSeparateWindow &&
+                !appEnvironment.navigationCoordinator.isMacPlayerFullScreenOverlay
             },
-            set: { appEnvironment.navigationCoordinator.isPlayerExpanded = $0 }
+            set: { newValue in
+                // Ignore the dismissal write-back when the sheet is swapped for
+                // the fullscreen overlay — the player stays expanded there.
+                if !newValue, appEnvironment.navigationCoordinator.isMacPlayerFullScreenOverlay {
+                    return
+                }
+                appEnvironment.navigationCoordinator.isPlayerExpanded = newValue
+            }
         )) {
             let size = expandedSheetSize(appEnvironment: appEnvironment)
             let lockAspect = sheetLockAspectRatio(appEnvironment: appEnvironment)