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".

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