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/pages/Settings.tsx

@@ -360,6 +360,7 @@ const CONTRIBUTORS = [
       'Settings: 3-state animation mode (Full / Reduced / Static) — replaces boolean reduce-animations toggle (PR #441)',
       'Tracks: Highly Rated rail and per-card star display, with cache layer for ndListSongs (PR #443)',
       'Random Mix: playlist-size picker (50/75/100/125/150) and filter-panel layout cleanup (PR #445)',
+      'Queue: optional "Preserve Play Next order" toggle — multiple Play Next inserts queue up behind each other instead of latest-on-top (PR #464)',
     ],
   },
 ] as const;
@@ -2552,6 +2553,22 @@ export default function Settings() {
                   <span className="toggle-track" />
                 </label>
               </div>
+
+              <div className="settings-toggle-row" style={{ marginTop: '0.75rem' }}>
+                <div>
+                  <div style={{ fontWeight: 500 }}>
+                    {t('settings.preservePlayNextOrder')}
+                  </div>
+                  <div style={{ fontSize: 12, color: 'var(--text-muted)' }}>
+                    {t('settings.preservePlayNextOrderDesc')}
+                  </div>
+                </div>
+                <label className="toggle-switch" aria-label={t('settings.preservePlayNextOrder')}>
+                  <input type="checkbox" checked={auth.preservePlayNextOrder}
+                    onChange={e => auth.setPreservePlayNextOrder(e.target.checked)} />
+                  <span className="toggle-track" />
+                </label>
+              </div>
             </div>
           </SettingsSubSection>