fix: 修复 macOS 桌面应用打包与元数据

将 macOS 桌面应用改为通用二进制并动态写入最低系统版本，避免 Intel Mac 无法启动。统一桌面应用名称与托盘展示，并补充测试确保相关行为稳定。

backend/cmd/desktop/metadata_test.go

@@ -0,0 +1,24 @@
+package main
+
+import "testing"
+
+func TestAboutMessage(t *testing.T) {
+	expected := "Nex\n\nAI Gateway - 统一的大模型 API 网关\n\nhttps://github.com/nex/gateway"
+	if got := aboutMessage(); got != expected {
+		t.Fatalf("aboutMessage() = %q, want %q", got, expected)
+	}
+}
+
+func TestDesktopMetadata(t *testing.T) {
+	if appName != "Nex" {
+		t.Fatalf("appName = %q, want %q", appName, "Nex")
+	}
+
+	if appTooltip != appName {
+		t.Fatalf("appTooltip = %q, want %q", appTooltip, appName)
+	}
+
+	if appAboutTitle != "关于 Nex" {
+		t.Fatalf("appAboutTitle = %q, want %q", appAboutTitle, "关于 Nex")
+	}
+}