feat(runbackup): opt-in auto-continue + manual continue on pre-update screen
The pre-update/uninstall full-screen backup now shows an "Automatically
continue the update when the backup finishes" checkbox (ticked by
default, so the seamless flow is unchanged). If unticked, the screen
stops on completion and offers a link to the new backup record plus a
"Continue the update" button, so the user can hand back to Joomla
manually — a reliable fallback when auto-continue does not fire.
Also drop the hard-to-read `small` text class from the progress screen.
Claude-Session: https://claude.ai/code/session_01WbGBN9VyRK61zczYWcCQ2i
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jmiller