Redesign macOS now-playing bar as centered Liquid Glass capsule

- Replace the full-width material overlay bar with a compact, centered
  capsule (Apple Music style) via a macOS-specific macOSCapsuleLayout
- Extend glassBackground() to use real Liquid Glass on macOS 26 with a
  material fallback on macOS 15-25 (mirrors the PlayerOverlayButton pattern)
- Remove the duplicate close button (close now comes only from the
  configurable button set)
- Keep the capsule visible in the main window while the separate player
  window is open (window mode), still hidden in sheet mode

Yattee/Views/Components/GlassBackground.swift

@@ -64,23 +64,23 @@ struct GlassBackgroundModifier: ViewModifier {
     }
 
     func body(content: Content) -> some View {
-        #if os(iOS)
-        if #available(iOS 26.0, *) {
+        #if os(iOS) || os(macOS)
+        if #available(iOS 26.0, macOS 26.0, *) {
             content.modifier(LiquidGlassModifier(style: style, shape: shape, colorScheme: colorScheme))
         } else {
             content.modifier(FallbackGlassModifier(shape: shape, material: fallbackMaterial, colorScheme: colorScheme))
         }
         #else
-        // macOS / tvOS don't have glassEffect, always use fallback material
+        // tvOS doesn't have glassEffect, always use fallback material
         content.modifier(FallbackGlassModifier(shape: shape, material: fallbackMaterial, colorScheme: colorScheme))
         #endif
     }
 }
 
-// MARK: - iOS 26+ Liquid Glass
+// MARK: - iOS 26+ / macOS 26+ Liquid Glass
 
-#if os(iOS)
-@available(iOS 26.0, *)
+#if os(iOS) || os(macOS)
+@available(iOS 26.0, macOS 26.0, *)
 private struct LiquidGlassModifier: ViewModifier {
     let style: GlassStyle
     let shape: GlassShape