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/GameHubContext.cs

@@ -0,0 +1,21 @@
+using Microsoft.AspNetCore.SignalR;
+
+namespace Shogi.Api.Application;
+
+/// <summary>
+/// Used to send signals to connected clients.
+/// </summary>
+public class GameHubContext(IHubContext<GameHub> context)
+{
+	public async Task Emit_SessionJoined(string sessionId)
+	{
+		var clients = context.Clients.Group(sessionId);
+		await clients.SendAsync("SessionJoined");
+	}
+
+	public async Task Emit_PieceMoved(string sessionId)
+	{
+		var clients = context.Clients.Group(sessionId);
+		await clients.SendAsync("PieceMoved");
+	}
+}