Add IsAvailable property to Inbound and update related logic

- Introduced a new boolean property, `IsAvailable`, to the `Inbound` entity to track the availability status of inbounds based on synchronization results.
- Updated the `SyncNodeCommandHandler` to mark inbounds as unavailable if they are not present in the latest synchronization but have existing configurations, preventing their deletion.
- Enhanced the `MarkUnavailable` method to set both `IsAvailable` and `IsPublished` to false, reflecting the new status accurately.
- Modified the frontend components to display the availability status of inbounds, ensuring users are informed of their current state.
- Updated tests to cover the new behavior regarding inbound availability and its impact on revocation processes.

backend/src/PnvPanel.Domain/Inbounds/Inbound.cs

@@ -16,6 +16,7 @@ public sealed class Inbound : Entity
     public string Remark { get; private set; } = string.Empty;
     public int Port { get; private set; }
     public bool IsPublished { get; private set; }
+    public bool IsAvailable { get; private set; } = true;
     public string? DisplayName { get; private set; }
     public IReadOnlyList<Guid> AllowedRoleIds { get; private set; } = [];
     public DateTimeOffset? LastSyncAt { get; private set; }
@@ -39,6 +40,7 @@ public sealed class Inbound : Entity
             Remark = remark,
             Port = port,
             IsPublished = false,
+            IsAvailable = true,
             LastSyncAt = DateTimeOffset.UtcNow,
         };
     }
@@ -59,4 +61,15 @@ public sealed class Inbound : Entity
     }
 
     public void Unpublish() => IsPublished = false;
+
+    /// <summary>
+    /// Инбаунд пропал на панели (не пришёл в очередной синхронизации), но по нему есть конфиги —
+    /// удалить запись нельзя (FK), поэтому помечаем недоступной вместо тихого "не опубликован":
+    /// UI показывает явный статус, а Revoke пропускает обращение к панели как заведомо мёртвое.
+    /// </summary>
+    public void MarkUnavailable()
+    {
+        IsAvailable = false;
+        IsPublished = false;
+    }
 }