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/ipc/ipc.go

@@ -109,8 +109,16 @@ func NewServer() (*Server, error) {
 	netType := paths.IPCNetwork()
 	addr := paths.IPCAddress()
 
-	// Clean up stale unix socket file (not needed for TCP).
+	// Guard against a second daemon instance on unix: probe the
+	// socket before removing it. If the dial succeeds, another
+	// daemon is alive and owns the socket - refuse to start so we
+	// don't silently orphan it. Only remove the file when the dial
+	// fails (stale socket from a crashed process).
 	if netType == "unix" {
+		if c, derr := net.Dial(netType, addr); derr == nil {
+			c.Close()
+			return nil, fmt.Errorf("daemon already running on %s", addr)
+		}
 		_ = os.Remove(addr)
 	}