Add a cancelable redirect handoff and drop the transitional migration status

When the server finishes starting, show "Jellyfin started successfully" with a
5-second "Redirecting in N…" countdown and a Cancel button instead of reloading
immediately. Cancel stops the countdown and the background refresh so the
startup output can be reviewed, and offers a "Continue to Jellyfin" button to
reload manually. The buttons use the web client's emby-button styling.

Also drop the transitional "Applying migrations" activity: it only showed
briefly while the pending migration set was read, or for the whole step when
nothing was pending, so startup now goes from "Preparing migrations" straight
into "Running migration X of Y".

Jellyfin.Server/ServerSetupApp/SetupServer.cs

@@ -262,7 +262,7 @@ public sealed class SetupServer : IDisposable
     /// Reports the current startup activity shown to all clients in the startup UI header.
     /// Only pass generic, non-identifying text from <see cref="StartupActivity"/>.
     /// </summary>
-    /// <param name="activity">A generic description such as <see cref="StartupActivity.ApplyingMigrations"/>.</param>
+    /// <param name="activity">A generic description such as <see cref="StartupActivity.PreparingMigrations"/>.</param>
     internal static void ReportActivity(string activity)
     {
         _currentActivity = activity;