fix(macos): 🐛 restore opaque window background for title bar (#2448)

Keep the first-frame-ready show flow from #1287, but stop clearing the
native window background so the macOS title bar no longer appears transparent.

Co-authored-by: Wine Fox <fox@ling.plus>

macos/Runner/MainFlutterWindow.swift

@@ -9,9 +9,9 @@ class MainFlutterWindow: NSWindow {
     self.contentViewController = flutterViewController
     self.setFrame(self.frame, display: true)
 
-    // 背景别用默认黑色
-    self.isOpaque = false
-    self.backgroundColor = .clear
+    // 使用系统窗口背景色，避免启动防闪时把标题栏/窗口背景透成透明
+    self.isOpaque = true
+    self.backgroundColor = .windowBackgroundColor
 
     RegisterGeneratedPlugins(registry: flutterViewController)