feat: split AutoDJ into a standalone playback feature (#1124)

* feat(playback): add transition-mode helper

Centralise the crossfade/AutoDJ/gapless mutual exclusivity in one place
instead of the scattered setter combinations across the toolbar, mini
player and settings. AutoDJ stays encoded as crossfade + trim-silence, so
the persisted flags and the audio engine are unchanged.

* feat(queue): split AutoDJ into its own toolbar button + playlist submenu

- AutoDJ becomes a standalone toolbar button (Blend icon) next to
  crossfade, driven by the shared transition-mode helper. The crossfade
  right-click popover drops the mode switch and keeps only the seconds
  slider.
- Save + load playlist collapse into one Playlist button opening a small
  submenu, freeing up toolbar space.
- queueToolbarStore gains a position-preserving rehydrate migration
  (legacy save/load -> playlist, autodj inserted after crossfade) with
  unit tests.
- Toolbar customizer and all 9 queue locales updated to match.

* feat(mini-player): standalone AutoDJ button, shared transition helper

Mirror the queue toolbar: AutoDJ gets its own Blend button, the crossfade
popover keeps only the seconds slider. The mini player now drives all
three transitions through a single additive `mini:set-transition-mode`
event handled by the shared helper, replacing the per-flag mini events.
Drop the now-dead crossfade-mode CSS.

* feat(settings): segmented track-transition picker, regroup playback

Replace the crossfade toggle + inner crossfade/AutoDJ switch with a single
Off | Gapless | Crossfade | AutoDJ segmented control (mirroring the
Normalization picker above), driven by the shared transition helper — the
mutual exclusivity now reads at a glance. Crossfade keeps its seconds
slider; AutoDJ shows its content-driven explainer. The block is regrouped
under "Track transitions" and "Queue behaviour" headings. Drops the now
unused crossfade/gapless description and not-available i18n keys across all
9 locales and adds the new transition strings.

* fix(queue): open the playlist submenu inward

The playlist submenu inherited the crossfade popover's right:0 anchor and,
sitting on the left of the toolbar, opened out under the main container.
Anchor it left:0 so it stays inside the queue panel. Update the toolbar
test for the new playlist submenu (save/load moved off the toolbar).

* feat(settings): box playback sub-sections into panels

Wrap Normalization, Track transitions and Queue behaviour each in their own
bordered panel with an accent uppercase header (new reusable .settings-group
classes), so the sections read as distinct blocks instead of one wall of
text. Drops the thin divider that separated them.

* docs: changelog, credits and what's new for AutoDJ standalone

Fold the standalone-AutoDJ changes into the existing AutoDJ entry in the
changelog and the in-app What's New, and add the credit (#1124).

src/components/QueuePanel.tsx

@@ -109,10 +109,7 @@ function QueuePanelHostOrSolo() {
   const crossfadeTrimSilence = useAuthStore(s => s.crossfadeTrimSilence);
   const gaplessEnabled = useAuthStore(s => s.gaplessEnabled);
   const infiniteQueueEnabled = useAuthStore(s => s.infiniteQueueEnabled);
-  const setCrossfadeEnabled = useAuthStore(s => s.setCrossfadeEnabled);
   const setCrossfadeSecs = useAuthStore(s => s.setCrossfadeSecs);
-  const setCrossfadeTrimSilence = useAuthStore(s => s.setCrossfadeTrimSilence);
-  const setGaplessEnabled = useAuthStore(s => s.setGaplessEnabled);
   const setInfiniteQueueEnabled = useAuthStore(s => s.setInfiniteQueueEnabled);
   const normalizationEngine = useAuthStore(s => s.normalizationEngine);
 
@@ -337,13 +334,10 @@ function QueuePanelHostOrSolo() {
             handleCopyQueueShare={handleCopyQueueShare}
             handleClear={handleClear}
             gaplessEnabled={gaplessEnabled}
-            setGaplessEnabled={setGaplessEnabled}
             crossfadeEnabled={crossfadeEnabled}
-            setCrossfadeEnabled={setCrossfadeEnabled}
+            crossfadeTrimSilence={crossfadeTrimSilence}
             crossfadeSecs={crossfadeSecs}
             setCrossfadeSecs={setCrossfadeSecs}
-            crossfadeTrimSilence={crossfadeTrimSilence}
-            setCrossfadeTrimSilence={setCrossfadeTrimSilence}
             infiniteQueueEnabled={infiniteQueueEnabled}
             setInfiniteQueueEnabled={setInfiniteQueueEnabled}
             t={t}