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.UI/Pages/Play/GameBoard/SpectatorGameBoard.razor

@@ -0,0 +1,29 @@
+@using Contracts.Types
+@using System.Net
+@inject ShogiApi ShogiApi
+
+<GameBoardPresentation IsSpectating="true"
+											 Perspective="WhichPlayer.Player2"
+											 Session="Session"
+											 OnClickJoinGame="OnClickJoinGame" />
+
+@code {
+	[Parameter]
+	[EditorRequired]
+	public Session Session { get; set; } = default!;
+
+	protected override void OnParametersSet()
+	{
+		base.OnParametersSet();
+		if (Session == null)
+		{
+			throw new ArgumentException($"{nameof(Session)} cannot be null.", nameof(Session));
+		}
+	}
+
+	async Task OnClickJoinGame()
+	{
+		var response = await ShogiApi.PatchJoinGame(Session.SessionId.ToString());
+		response.EnsureSuccessStatusCode();
+	}
+}