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/utils/formatHumanDuration.ts

@@ -0,0 +1,11 @@
+import i18n from '../i18n';
+
+/** Totals / statistics: localized "N hours M minutes" (not track mm:ss). */
+export function formatHumanHoursMinutes(seconds: number): string {
+  const h = Math.floor(seconds / 3600);
+  const m = Math.floor((seconds % 3600) / 60);
+  if (h > 0) {
+    return i18n.t('common.durationHoursMinutes', { hours: h.toLocaleString(), minutes: m });
+  }
+  return i18n.t('common.durationMinutesOnly', { minutes: m });
+}