添加时间格式转换器和命名空间引用

在 `App.xaml` 中添加了对 `chatclient.Data` 命名空间的引用。
在 `ChatDataModel.cs` 中移除不必要的 `using` 语句，添加对 `System.Windows` 和 `System.Windows.Media.Imaging` 的引用，并为 `ConvertBack` 方法添加文档注释。
在 `MainWindow.xaml` 中定义了 `TimeFormatConverter` 资源，以增强时间显示功能。

chatclient/App.xaml

@@ -3,6 +3,7 @@
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:materialDesign="http://materialdesigninxaml.net/winfx/xaml/themes"
             xmlns:local="clr-namespace:chatclient"
+            xmlns:data="clr-namespace:chatclient.Data"
             StartupUri="MainWindow.xaml">
     <Application.Resources>
         <ResourceDictionary>

chatclient/Data/ChatDataModel.cs

@@ -1,13 +1,6 @@
-using System;
+using System.Windows.Media;
-using System.Collections.Generic;
-using System.Linq;
-using System.Text;
-using System.Threading.Tasks;
-using System.Windows.Media;
 using System.Windows;
 using System.Windows.Media.Imaging;
-using System.Windows.Controls;
-using System.Windows.Input;
 using System.Globalization;
 using System.Windows.Data;
 
@@ -62,9 +55,6 @@ namespace chatclient.Data
         public string? UserName { get; set; }
         public string? UserPassword { get; set; }
     }
-    /// <summary>
-    /// 时间格式转换器类
-    /// </summary>
     public class TimeFormatConverter : IValueConverter
     {
         public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
@@ -81,7 +71,6 @@ namespace chatclient.Data
                 }
                 else if (timestamp.Kind == DateTimeKind.Unspecified)
                 {
-                    // 假设未指定时间是UTC时间（常见实践）
                     localTime = DateTime.SpecifyKind(timestamp, DateTimeKind.Utc).ToLocalTime();
                 }
                 else
@@ -127,7 +116,15 @@ namespace chatclient.Data
             }
             return value;
         }
-
+        /// <summary>
+        /// 将格式化的时间字符串转换回DateTime对象
+        /// </summary>
+        /// <param name="value"></param>
+        /// <param name="targetType"></param>
+        /// <param name="parameter"></param>
+        /// <param name="culture"></param>
+        /// <returns></returns>
+        /// <exception cref="NotImplementedException"></exception>
         public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
         {
             throw new NotImplementedException();

chatclient/MainWindow.xaml

@@ -10,6 +10,9 @@
         mc:Ignorable="d"
         Title="ChatWindow" Height="450" Width="800" MinHeight="240" MinWidth="380"
         Style="{StaticResource MaterialDesignWindow}" Closed="MainWindow_Closed" Loaded="MainWindow_Loaded">
+    <Window.Resources>
+        <data:TimeFormatConverter x:Key="TimeConverter"/>
+    </Window.Resources>
     <Grid>
         <materialDesign:Card>
             <TabControl x:Name="TabControl" VerticalContentAlignment="Bottom" materialDesign:ColorZoneAssist.Mode="PrimaryMid" Style="{StaticResource MaterialDesignNavigationRailTabControl}">