Configs that predate posterRatingProvider were resolved to 'top' whenever a
Top Poster key existed, so users who never enabled rating posters got them
anyway while the UI showed RPDB selected. Infer from the saved poster pattern
instead, which is the only record of an actual choice, and fall back to a new
explicit 'none' provider when there is no pattern.

Adds None to the provider dropdown, centralises pattern resolution in
resolvePosterPattern/resolveThumbnailPattern so every call site honours it,
and hides the per-catalog rating poster stars when the provider is None.