Restyle the startup/migration holding page to match the Jellyfin dark theme,
with the inline wordmark logo, a gradient spinner and a recolored startup log
tree, and move the Morestachio template rendering into a reusable
StartupUiRenderer.
Add a curated, non-identifying "current activity" line to the always-visible
header (for example "Initializing server" or "Running migration X of Y"),
reported from the startup flow and the migration service so it never leaks
server details to unauthenticated clients. Move the log download into a
"Download logs" link in the log panel header, and show only the header, with
no log hints, to non-local clients.
