add: 新建对话和切换对话功能

Frontend/Models/Chat.cs

@@ -0,0 +1,12 @@
+namespace Frontend.Models;
+
+public class Chat
+{
+    public Guid Guid { get; } = Guid.NewGuid();
+
+    public string Title { get; set; } = string.Empty;
+    
+    public bool Selected { get; set; }
+
+    public List<ChatMessage> Messages { get; } = new();
+}