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/StartupActivity.cs

@@ -21,9 +21,6 @@ public static class StartupActivity
     /// <summary>Preparing the system for migrations (e.g. taking safety backups).</summary>
     public const string PreparingMigrations = "Preparing migrations";
 
-    /// <summary>Applying database/system migrations without a known count.</summary>
-    public const string ApplyingMigrations = "Applying migrations";
-
     /// <summary>Restoring from a backup.</summary>
     public const string RestoringBackup = "Restoring backup";