Allow UI to delete sessions

Shogi.UI/Shared/ShogiApi.cs

@@ -70,5 +70,10 @@ public class ShogiApi(HttpClient httpClient)
 		return httpClient.PatchAsync(Relative($"Sessions/{name}/Join"), null);
 	}
 
+	public Task<HttpResponseMessage> DeleteSession(Guid sessionId)
+	{
+		return httpClient.DeleteAsync(Relative($"Sessions/{sessionId}"));
+	}
+
 	private static Uri Relative(string path) => new(path, UriKind.Relative);
 }