fix(statistics): cache Subsonic stat fetches and localize duration units

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.

src/locales/de.ts

@@ -361,6 +361,8 @@ export const deTranslation = {
     updaterAurHint: 'Update über AUR installieren:',
     updaterErrorMsg: 'Download fehlgeschlagen',
     updaterRetryBtn: 'Erneut versuchen',
+    durationHoursMinutes: '{{hours}} Std. {{minutes}} Min.',
+    durationMinutesOnly: '{{minutes}} Min.',
     updaterOpenGitHub: 'Auf GitHub öffnen',
   },
   settings: {