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/de.ts

@@ -891,6 +891,8 @@ export const deTranslation = {
     notWithCrossfade: 'Nicht verfügbar wenn Crossfade aktiv ist',
     gapless: 'Nahtlose Wiedergabe',
     gaplessDesc: 'Nächsten Track vorpuffern um Lücken zwischen Songs zu vermeiden',
+    preservePlayNextOrder: '„Als Nächstes"-Reihenfolge bewahren',
+    preservePlayNextOrderDesc: 'Neu hinzugefügte „Als Nächstes"-Titel reihen sich hinten an statt sich vorn einzuschieben.',
     trackPreviewsTitle: 'Track-Vorschau',
     trackPreviewsToggle: 'Track-Vorschau aktivieren',
     trackPreviewsDesc: 'Inline Play- und Vorschau-Buttons in den Tracklisten anzeigen für eine kurze Hörprobe mitten im Song.',