Keep macOS player transport buttons visible but disabled when unavailable

The play-next and Picture-in-Picture buttons were folded into the view
condition (hasNext / isPiPPossible), so they popped in and out of the
control bar and shifted the surrounding layout. Always render them (when
their callback is wired) and gate availability with .disabled() instead,
mirroring the existing play-previous button.

Yattee/Views/Player/macOS/MacOSControlBar.swift

@@ -180,8 +180,8 @@ struct MacOSControlBar: View {
             .disabled(isTransportDisabled)
             .opacity(isTransportDisabled ? 0.3 : 1.0)
 
-            // Play next (if queue has items)
-            if let onPlayNext, playerState.hasNext {
+            // Play next (disabled when no next in queue)
+            if let onPlayNext {
                 Button {
                     playNextTapCount += 1
                     Task { await onPlayNext() }
@@ -193,6 +193,7 @@ struct MacOSControlBar: View {
                         .symbolEffect(.bounce.down.byLayer, options: .nonRepeating, value: playNextTapCount)
                 }
                 .buttonStyle(MacOSControlButtonStyle())
+                .disabled(!playerState.hasNext)
             }
         }
     }
@@ -366,7 +367,7 @@ struct MacOSControlBar: View {
             }
 
             // PiP
-            if let onTogglePiP, playerState.isPiPPossible {
+            if let onTogglePiP {
                 Button {
                     onTogglePiP()
                 } label: {
@@ -376,6 +377,7 @@ struct MacOSControlBar: View {
                         .contentShape(Rectangle())
                 }
                 .buttonStyle(MacOSControlButtonStyle())
+                .disabled(!playerState.isPiPPossible)
             }
         }
     }