feat: 单实例锁与窗口恢复机制

- GUI 单实例锁: 新增 internal/instance 包,首实例监听专用端点,
  第二实例发送 focus 信号后静默退出 (Unix: gui.sock, Win: 18924)
- 守护进程弱锁修复: NewServer() 在 os.Remove 前先 net.Dial 探测,
  有活进程时拒绝启动,避免静默抢占 socket 导致孤儿进程
- macOS .app 重开恢复: 通过 class_addMethod 给 GLFWApplicationDelegate
  注册 applicationShouldHandleReopen:hasVisibleWindows: 回调,
  绕过 GLFW 直接用 Cocoa makeKeyAndOrderFront: 显示窗口
- 三平台覆盖: macOS/Linux 用 unix socket, Windows 用 TCP 端口

internal/ui/dock_callback_darwin.go

@@ -0,0 +1,18 @@
+//go:build darwin
+
+package ui
+
+import "C"
+
+// onAppActive is called from the Cocoa main thread when the app
+// becomes active (e.g. the user re-opens the .app bundle while the
+// process is still running).  It is set once in Run() before the
+// event loop starts, so no synchronisation is needed.
+var onAppActive func()
+
+//export cmAppBecameActive
+func cmAppBecameActive() {
+	if onAppActive != nil {
+		onAppActive()
+	}
+}