fix(client): 单实例锁改用 TCP loopback socket

文件锁在 Windows 上行为不一致，改用绑定本地端口（54317）作为互斥锁，
进程退出时 OS 自动释放，Windows/macOS 均可靠生效。

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

client/lib/core/platform/single_instance.dart

@@ -1,11 +1,13 @@
 import 'dart:io';
 
 import 'package:flutter/foundation.dart';
-import 'package:path_provider/path_provider.dart';
 
-// Keep the RAF open so the OS lock stays alive for the lifetime of the process.
-// ignore: unused_element
-RandomAccessFile? _raf;
+// Keep the server socket alive for the lifetime of the process.
+// The OS releases the port automatically when the process exits.
+ServerSocket? _server;
+
+// Port unique to this app; chosen from the private range 49152–65535.
+const _kLockPort = 54317;
 
 /// Returns true if this process successfully acquired the single-instance lock.
 /// Returns false if another instance is already running.
@@ -15,11 +17,11 @@ Future<bool> acquire() async {
   if (!Platform.isWindows && !Platform.isMacOS) return true;
 
   try {
-    final dir = await getApplicationSupportDirectory();
-    final lockFile = File('${dir.path}/jiu.lock');
-    final raf = await lockFile.open(mode: FileMode.write);
-    await raf.lock(FileLock.exclusive);
-    _raf = raf;
+    _server = await ServerSocket.bind(
+      InternetAddress.loopbackIPv4,
+      _kLockPort,
+      shared: false,
+    );
     return true;
   } catch (_) {
     return false;