feat(client/macos): 开机自启用 SMAppService + LoginItem helper(学 Tailscale)

根因:launch_at_startup 0.5.1 在 macOS 无自带实现,需宿主接 MethodChannel,而我们 Runner
从没实现 → enable() 抛 MissingPluginException 被 try/catch 吞掉 → 登录项从未注册,自启失败。
学 Tailscale(LoginItemHelper-macsys + SMAppService):
- 新增无界面 LoginItem helper(macos/login_helper,LSBackgroundOnly),登录时带 --autostart
  拉起主 app(→隐藏到托盘);
- build_login_helper.sh 构建期编译+签名+嵌入 Contents/Library/LoginItems/(新 Run Script 阶段);
- MainFlutterWindow 接 launch_at_startup MethodChannel → SMAppService.loginItem 注册/注销。
构建通过:helper 嵌入+签名,主 app --deep --strict 校验过。Windows 自启(schtasks)不变。

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

client/macos/Runner.xcodeproj/project.pbxproj

@@ -342,6 +342,7 @@
 				3399D490228B24CF009A79C7 /* ShellScript */,
 				D3610C37E667F89FD1D27249 /* [CP] Embed Pods Frameworks */,
 				A17B79EA2FE50746001ABF28 /* Copy System Extensions */,
+				FACE0FF0FACE0FF0FACE0002 /* Embed Login Helper */,
 			);
 			buildRules = (
 			);
@@ -371,6 +372,20 @@
 			shellPath = /bin/sh;
 			shellScript = "bash \"${SRCROOT}/sign_libbox.sh\"\n";
 		};
+		FACE0FF0FACE0FF0FACE0002 /* Embed Login Helper */ = {
+			isa = PBXShellScriptBuildPhase;
+			buildActionMask = 2147483647;
+			files = (
+			);
+			inputPaths = (
+			);
+			name = "Embed Login Helper";
+			outputPaths = (
+			);
+			runOnlyForDeploymentPostprocessing = 0;
+			shellPath = /bin/sh;
+			shellScript = "bash \"${SRCROOT}/build_login_helper.sh\"\n";
+		};
 		A17B79DE2FE50745001ABF28 /* PacketTunnel */ = {
 			isa = PBXNativeTarget;
 			buildConfigurationList = A17B79EF2FE50746001ABF28 /* Build configuration list for PBXNativeTarget "PacketTunnel" */;