Ground work for Wino Calendar. (#475)

Wino Calendar abstractions.

Wino.Core.ViewModels/Data/MergedAccountProviderDetailViewModel.cs

@@ -0,0 +1,36 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using CommunityToolkit.Mvvm.ComponentModel;
+using Wino.Core.Domain.Entities.Mail;
+using Wino.Core.Domain.Interfaces;
+using Wino.Mail.ViewModels.Data;
+
+namespace Wino.Core.ViewModels.Data
+{
+    public partial class MergedAccountProviderDetailViewModel : ObservableObject, IAccountProviderDetailViewModel
+    {
+        public List<AccountProviderDetailViewModel> HoldingAccounts { get; }
+        public MergedInbox MergedInbox { get; }
+
+        public string AccountAddresses => string.Join(", ", HoldingAccounts.Select(a => a.Account.Address));
+
+        public Guid StartupEntityId => MergedInbox.Id;
+
+        public string StartupEntityTitle => MergedInbox.Name;
+
+        public int Order => 0;
+
+        public IProviderDetail ProviderDetail { get; set; }
+
+        public string StartupEntityAddresses => AccountAddresses;
+
+        public int HoldingAccountCount => HoldingAccounts.Count;
+
+        public MergedAccountProviderDetailViewModel(MergedInbox mergedInbox, List<AccountProviderDetailViewModel> holdingAccounts)
+        {
+            MergedInbox = mergedInbox;
+            HoldingAccounts = holdingAccounts;
+        }
+    }
+}