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/styles/components/mini-player-window.css

@@ -240,39 +240,6 @@
   cursor: pointer;
 }
 
-.mini-player__crossfade-modes {
-  display: flex;
-  gap: 4px;
-}
-
-.mini-player__crossfade-mode {
-  flex: 1 1 0;
-  min-width: 0;
-  padding: 5px 6px;
-  font-size: 11px;
-  font-weight: 600;
-  border: 1px solid var(--border-subtle);
-  border-radius: 6px;
-  background: transparent;
-  color: var(--text-secondary);
-  cursor: pointer;
-  white-space: nowrap;
-  overflow: hidden;
-  text-overflow: ellipsis;
-  transition: background 0.12s ease, color 0.12s ease, border-color 0.12s ease;
-}
-
-.mini-player__crossfade-mode:hover {
-  border-color: color-mix(in srgb, var(--accent) 40%, var(--border-subtle));
-  color: var(--text-primary);
-}
-
-.mini-player__crossfade-mode.active {
-  background: var(--accent);
-  border-color: var(--accent);
-  color: var(--bg-app);
-}
-
 .mini-player__volume-pct {
   font-size: 10px;
   color: var(--text-muted);