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/zh.ts

@@ -357,6 +357,8 @@ export const zhTranslation = {
     updaterAurHint: '通过 AUR 安装更新：',
     updaterErrorMsg: '下载失败',
     updaterRetryBtn: '重试',
+    durationHoursMinutes: '{{hours}}小时{{minutes}}分钟',
+    durationMinutesOnly: '{{minutes}}分钟',
     updaterOpenGitHub: '在 GitHub 上打开',
   },
   settings: {
@@ -739,7 +741,7 @@ export const zhTranslation = {
     statAlbums: '专辑',
     statSongs: '歌曲',
     statGenres: '流派',
-    statPlaytime: '总播放时长',
+    statPlaytime: '音频总时长',
     genreInsights: '流派洞察',
     formatDistribution: '格式分布',
     formatSample: '{{n}} 首曲目的样本',