feat: title bar hover to show/hide

- Title bar hidden by default (titleVisibility + titlebarAppearsTransparent)
- Mouse enter top 28pt area: show title bar
- Mouse exit: hide title bar
- Decouple title visibility from toolbar state

Sources/MiniPlayerApp.swift

@@ -319,18 +319,35 @@ struct PlayerContentView: View {
             }
         }
         .onAppear {
-            // 初始状态：标题随toolbar一起隐藏
-            NSApplication.shared.windows.first?.titleVisibility = .hidden
+            // 初始状态：标题栏完全隐藏
+            if let win = NSApplication.shared.windows.first {
+                win.titleVisibility = .hidden
+                win.titlebarAppearsTransparent = true
+            }
         }
         .onChange(of: showToolbar) { _, newValue in
-            // 同步控制窗口标题的显示/隐藏（与toolbar联动）
-            NSApplication.shared.windows.first?.titleVisibility = newValue ? .visible : .hidden
             if newValue {
                 resetHideTimer()
             } else {
                 hideTimer?.invalidate()
             }
         }
+        .overlay(alignment: .top) {
+            // 标题栏悬停检测：鼠标进入显示标题，退出隐藏
+            Color.clear
+                .frame(height: 28)
+                .contentShape(Rectangle())
+                .onHover { hovering in
+                    let win = NSApplication.shared.windows.first
+                    if hovering {
+                        win?.titleVisibility = .visible
+                        win?.titlebarAppearsTransparent = false
+                    } else {
+                        win?.titleVisibility = .hidden
+                        win?.titlebarAppearsTransparent = true
+                    }
+                }
+        }
         .onDisappear {
             hideTimer?.invalidate()
         }