Add Liquid Glass effect to player controls bar

Refactor controls bar background styling with platform-specific effects:
- Add Liquid Glass effect for iOS 26+ using new glassEffect API
- Fallback to ultraThinMaterial for iOS 15-25
- Fallback to blurred black overlay for iOS 14 and earlier
- Extract background logic into reusable applyControlsBackground modifier
- Adjust controls bar vertical offset for better alignment

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Shared/Modifiers/PlayerOverlayModifier.swift

@@ -22,7 +22,7 @@ struct PlayerOverlayModifier: ViewModifier {
     @ViewBuilder var overlay: some View {
         Group {
             ControlsBar(fullScreen: .constant(false), expansionState: $expansionState, playerBar: true)
-                .offset(x: expansionState == .mini && !controlsWhenMinimized ? 10 : 0, y: 0)
+                .offset(x: expansionState == .mini && !controlsWhenMinimized ? 10 : 0, y: -3)
                 .frame(maxWidth: maxWidth, alignment: .trailing)
                 .onAppear {
                     if playerButtonIsExpanded {