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.test.tsx

@@ -31,6 +31,7 @@ vi.mock('@/utils/orbitBulkGuard', () => ({
 }));
 
 import QueuePanel from './QueuePanel';
+import { fireEvent } from '@testing-library/react';
 import { renderWithProviders } from '@/test/helpers/renderWithProviders';
 import { usePlayerStore } from '@/store/playerStore';
 import { useAuthStore } from '@/store/authStore';
@@ -164,15 +165,28 @@ describe('QueuePanel — display mode', () => {
 });
 
 describe('QueuePanel — toolbar', () => {
-  it('exposes Shuffle / Save Playlist / Load Playlist / Share Queue / Clear via aria-label', () => {
+  it('exposes Shuffle / Playlist / Share Queue / Clear / AutoDJ via aria-label', () => {
     const tracks = makeTracks(3);
     seedQueue(tracks, { index: 0, currentTrack: tracks[0] });
     const { getByLabelText } = renderWithProviders(<QueuePanel />);
     expect(getByLabelText('Shuffle queue')).toBeInTheDocument();
-    expect(getByLabelText('Save Playlist')).toBeInTheDocument();
-    expect(getByLabelText('Load Playlist')).toBeInTheDocument();
+    expect(getByLabelText('Playlist')).toBeInTheDocument();
     expect(getByLabelText('Copy queue share link')).toBeInTheDocument();
     expect(getByLabelText('Clear queue')).toBeInTheDocument();
+    expect(getByLabelText('AutoDJ')).toBeInTheDocument();
+  });
+
+  it('Save and Load live inside the Playlist submenu, not directly on the toolbar', () => {
+    const tracks = makeTracks(3);
+    seedQueue(tracks, { index: 0, currentTrack: tracks[0] });
+    const { getByLabelText, container } = renderWithProviders(<QueuePanel />);
+    // The submenu is closed initially.
+    expect(container.querySelector('.queue-menu')).toBeNull();
+    fireEvent.click(getByLabelText('Playlist'));
+    const menu = container.querySelector('.queue-menu');
+    expect(menu).not.toBeNull();
+    expect(menu?.textContent).toContain('Save Playlist');
+    expect(menu?.textContent).toContain('Load Playlist');
   });
 
   it('Shuffle button is disabled when the queue has fewer than 2 tracks', () => {