Pace IGDB requests; rewrite the new help text

IGDB allows 4 requests a second, and an import searching three entries
at once (plus detail lookups) went over it. Every IGDB request now waits
at least 300 ms after the last one, and a 429 is retried after 1, 2 and
4 seconds before giving up.

The import dialog, ranking editor and their error messages are reworded
in plainer language, and talk about "spots" instead of "places".

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/store/lists.go

@@ -176,7 +176,7 @@ func (s *Store) UpdateList(ctx context.Context, l List) (List, error) {
 	if last, err := s.MaxRank(ctx, l.ID); err != nil {
 		return l, err
 	} else if l.MaxItems < last {
-		return l, invalid("An entry sits at #%d; move it up or close the gaps before shrinking the list to %d", last, l.MaxItems)
+		return l, invalid("Something's at #%d, so the list can't be smaller than that. Move it up or close the gaps first.", last)
 	}
 	if l.Category != cur.Category && cur.ItemCount > 0 {
 		return l, invalid("Can't change the category of a list that has items")