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/nb.ts

@@ -878,6 +878,8 @@ export const nbTranslation = {
     notWithCrossfade: 'Ikke tilgjengelig mens Crossfade er aktiv',
     gapless: 'Gapless avspilling',
     gaplessDesc: 'Forhåndsbuffer neste spor for å eliminere mellomrom mellom sanger',
+    preservePlayNextOrder: 'Behold "Spill neste"-rekkefølge',
+    preservePlayNextOrderDesc: 'Nye "Spill neste"-elementer havner bak de eksisterende i stedet for å snike seg foran.',
     trackPreviewsTitle: 'Sporforhåndsvisning',
     trackPreviewsToggle: 'Aktiver sporforhåndsvisning',
     trackPreviewsDesc: 'Vis innebygde Spill- og Forhåndsvisning-knapper i sporlister for en kort smakebit fra midten av sangen.',