Add 7-minute TTL caches (same idea as rating prefetch) for overview
album strips, random-song format sample, and paginated library
aggregates; key by server and music folder.
Introduce formatHumanHoursMinutes with common.duration* strings in all
locales for Statistics and playlist duration labels.
Refresh RU/ZH statistics and nav wording; fix zh playtime label.
Maxim Isaev