Route remaining huh forms through the base16 palette theme

Three huh forms still bypassed the shared uiform.Theme() introduced in PR #1542: the auto-update prompt and OPF pre-push prompt pinned huh.ThemeDracula, and the rewind overwrite confirm used huh's default theme. Switch all three to uiform.New so they pick up the base16 palette and accessibility handling consistently.

Drop the now-derivable accessible param from askOPFPrompt and remove the orphaned isAccessibleMode helper (its only remaining caller was the rewind form).