修复 CNC 执行中切换用户权限守卫失效（P1-5/P1-7）

PermissionService.CanSwitchUser 原本读 SystemState.OperationMode，但该字段
从未被写成非 Idle（唯一写入点 RecipeService 仍写 Idle），导致"CNC 执行中
禁止切换用户"从未真正生效。改为直接读 AppStateService.CncExecutionState.IsRunning，
这是 CncExecutionService 在开始/暂停/结束时真实维护的状态。
RibbonStatusAreaViewModel 同步改为订阅 CncExecutionStateChanged。

MainViewportService.IsCncRunning 与 CncExecutionState 仍是两套独立状态，
保持不变：SetCncRunning 内部的互斥检查（与检测池模式双向互斥）需要原子性，
直接改为读 CncExecutionState 会引入竞态窗口，不在本次修复范围内。

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

XplorePlane/Services/Security/PermissionService.cs

@@ -151,14 +151,7 @@ namespace XplorePlane.Services.Security
         public event EventHandler<RoleChangedEventArgs> RoleChanged;
 
         /// <inheritdoc />
-        public bool CanSwitchUser
-        {
-            get
-            {
-                var systemState = _appStateService.SystemState;
-                return systemState.OperationMode == OperationMode.Idle;
-            }
-        }
+        public bool CanSwitchUser => !_appStateService.CncExecutionState.IsRunning;
 
         public PermissionService(
             IEventAggregator eventAggregator,

XplorePlane/ViewModels/Main/RibbonStatusAreaViewModel.cs

@@ -62,8 +62,8 @@ namespace XplorePlane.ViewModels.Main
             _eventAggregator.GetEvent<RoleChangedEvent>()
                 .Subscribe(OnRoleChanged, ThreadOption.UIThread);
 
-            // Subscribe to SystemState changes to update CanSwitchUser
-            _appStateService.SystemStateChanged += OnSystemStateChanged;
+            // Subscribe to CncExecutionState changes to update CanSwitchUser
+            _appStateService.CncExecutionStateChanged += OnCncExecutionStateChanged;
 
             // Initialize from current state
             InitializeFromCurrentState();
@@ -84,7 +84,7 @@ namespace XplorePlane.ViewModels.Main
             CurrentRoleDisplayName = MapRoleToDisplayName(payload.NewRole);
         }
 
-        private void OnSystemStateChanged(object sender, StateChangedEventArgs<SystemState> e)
+        private void OnCncExecutionStateChanged(object sender, StateChangedEventArgs<XplorePlane.Models.Main.CncExecutionState> e)
         {
             CanSwitchUser = _permissionService.CanSwitchUser;
             SwitchUserCommand.RaiseCanExecuteChanged();