feat(queue): preserve Play Next order toggle (#464)

* feat(queue): add preservePlayNextOrder setting + playNext store action

- New Track.playNextAdded flag (analogous to autoAdded / radioAdded).
  Stale flags behind queueIndex are harmless — only forward streak scan.
- New playerStore action playNext(tracks): tags incoming tracks and
  delegates to enqueueAt for unified undo + server sync.
- New authStore boolean preservePlayNextOrder (default false). When on,
  playNext appends behind the existing Play-Next streak (Spotify-style)
  instead of inserting directly after the current track.

* refactor(context-menu): centralise Play Next; add Settings toggle + i18n

- Replace 3 inline splice/enqueueAt call sites in ContextMenu with the
  new playNext action. Side-benefit: the single-song path now goes
  through enqueueAt and gets undo + queue sync (previously missing).
- Settings → Audio → Playback: new toggle below Gapless.
- 8 locales: preservePlayNextOrder + preservePlayNextOrderDesc.

* docs(contributors): credit + changelog entry for #464

src/locales/zh.ts

@@ -873,6 +873,8 @@ export const zhTranslation = {
     notWithCrossfade: '交叉淡入淡出开启时不可用',
     gapless: '无缝播放',
     gaplessDesc: '预缓冲下一首曲目以消除歌曲间的间隙',
+    preservePlayNextOrder: '保留"下一首播放"顺序',
+    preservePlayNextOrderDesc: '新添加的"下一首播放"项目排在现有项目之后，而不是插到前面。',
     trackPreviewsTitle: '曲目预览',
     trackPreviewsToggle: '启用曲目预览',
     trackPreviewsDesc: '在曲目列表中显示内联播放与预览按钮，可快速试听歌曲中段。',