Replace custom socket implementation with SignalR.

Replace MSAL and custom cookie auth with Microsoft.Identity.EntityFramework
Also some UI redesign to accommodate different login experience.

Shogi.Api/Application/GameHub.cs

@@ -0,0 +1,19 @@
+using Microsoft.AspNetCore.SignalR;
+
+namespace Shogi.Api.Application;
+
+/// <summary>
+/// Used to receive signals from connected clients.
+/// </summary>
+public class GameHub : Hub
+{
+	public Task Subscribe(string sessionId)
+	{
+		return this.Groups.AddToGroupAsync(this.Context.ConnectionId, sessionId);
+	}
+
+	public Task Unsubscribe(string sessionId)
+	{
+		return this.Groups.RemoveFromGroupAsync(this.Context.ConnectionId, sessionId);
+	}
+}