✨ feat(*): 添加管理员解锁与 SFTP 探活能力

* 新增管理员解锁弹窗、手动操作权限控制与倒计时状态
* 支持系统设置页测试 SFTP 连接，并在启动时执行探活
* 补充设计时服务、全局异常兜底与相关单元测试

tests/AxiOmron.PcbCheck.Tests/SystemSettingViewModelTests.cs

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+using AxiOmron.PcbCheck.Models;
+using AxiOmron.PcbCheck.Options;
+using AxiOmron.PcbCheck.Services.Interfaces;
+using AxiOmron.PcbCheck.ViewModels;
+
+namespace AxiOmron.PcbCheck.Tests;
+
+/// <summary>
+/// 验证系统设置页视图模型中的 SFTP 测试连接行为。
+/// </summary>
+public sealed class SystemSettingViewModelTests
+{
+    /// <summary>
+    /// 当 SFTP 测试成功时，应更新成功状态文本。
+    /// </summary>
+    /// <returns>异步测试任务。</returns>
+    [Fact]
+    public async Task TestSftpConnectionAsync_ShouldReportSuccess_WhenConnectionSucceeds()
+    {
+        var configService = new FakeAppConfigService();
+        var sftpLookupService = new FakeSftpLookupService
+        {
+            TestOutcome = new SftpConnectionTestOutcome
+            {
+                IsSuccess = true,
+                RootPathAccessible = true,
+                StatusMessage = "SFTP 连接成功，根目录可访问。"
+            }
+        };
+        var viewModel = new SystemSettingViewModel(configService, sftpLookupService);
+
+        await viewModel.TestSftpConnectionCommand.ExecuteAsync(null);
+
+        Assert.Equal("SFTP 连接成功，根目录可访问。", viewModel.StatusMessage);
+    }
+
+    /// <summary>
+    /// 当 SFTP 测试失败时，应将失败原因展示到状态文本。
+    /// </summary>
+    /// <returns>异步测试任务。</returns>
+    [Fact]
+    public async Task TestSftpConnectionAsync_ShouldReportFailure_WhenConnectionFails()
+    {
+        var configService = new FakeAppConfigService();
+        var sftpLookupService = new FakeSftpLookupService
+        {
+            TestOutcome = new SftpConnectionTestOutcome
+            {
+                IsSuccess = false,
+                IsSystemError = true,
+                StatusMessage = "SFTP 连接失败: timeout"
+            }
+        };
+        var viewModel = new SystemSettingViewModel(configService, sftpLookupService);
+
+        await viewModel.TestSftpConnectionCommand.ExecuteAsync(null);
+
+        Assert.Equal("SFTP 连接失败: timeout", viewModel.StatusMessage);
+    }
+
+    private sealed class FakeAppConfigService : IAppConfigService
+    {
+        public AppConfig Config { get; } = new();
+
+        public AppConfig Load()
+        {
+            return Config;
+        }
+
+        public void Save(AppConfig config)
+        {
+            ArgumentNullException.ThrowIfNull(config);
+        }
+
+        public string GetConfigPath()
+        {
+            return "appConfig.json";
+        }
+    }
+
+    private sealed class FakeSftpLookupService : ISftpLookupService
+    {
+        public SftpConnectionTestOutcome TestOutcome { get; set; } = new();
+
+        public Task<SftpCheckOutcome> CheckFileAsync(string barcode, CancellationToken cancellationToken)
+        {
+            throw new NotSupportedException();
+        }
+
+        public Task<SftpConnectionTestOutcome> TestConnectionAsync(SftpOptions? options, CancellationToken cancellationToken)
+        {
+            return Task.FromResult(TestOutcome);
+        }
+    }
+}