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/utils/playback/playbackTransition.test.ts

@@ -0,0 +1,85 @@
+import { describe, expect, it, beforeEach } from 'vitest';
+import { useAuthStore } from '../../store/authStore';
+import {
+  getTransitionMode,
+  transitionFlagsFor,
+  setTransitionMode,
+  type TransitionMode,
+} from './playbackTransition';
+
+const MODES: TransitionMode[] = ['none', 'gapless', 'crossfade', 'autodj'];
+
+describe('getTransitionMode', () => {
+  it('maps each flag combination to its mode', () => {
+    expect(getTransitionMode({ gaplessEnabled: false, crossfadeEnabled: false, crossfadeTrimSilence: false })).toBe('none');
+    expect(getTransitionMode({ gaplessEnabled: true, crossfadeEnabled: false, crossfadeTrimSilence: false })).toBe('gapless');
+    expect(getTransitionMode({ gaplessEnabled: false, crossfadeEnabled: true, crossfadeTrimSilence: false })).toBe('crossfade');
+    expect(getTransitionMode({ gaplessEnabled: false, crossfadeEnabled: true, crossfadeTrimSilence: true })).toBe('autodj');
+  });
+
+  it('treats trim-silence as AutoDJ only while crossfade is on', () => {
+    // Stale trim flag with crossfade off is still "none", not AutoDJ.
+    expect(getTransitionMode({ gaplessEnabled: false, crossfadeEnabled: false, crossfadeTrimSilence: true })).toBe('none');
+  });
+
+  it('is total — gapless wins if both gapless and crossfade are set', () => {
+    expect(getTransitionMode({ gaplessEnabled: true, crossfadeEnabled: true, crossfadeTrimSilence: true })).toBe('gapless');
+  });
+});
+
+describe('transitionFlagsFor', () => {
+  it('round-trips through getTransitionMode for every mode', () => {
+    for (const mode of MODES) {
+      expect(getTransitionMode(transitionFlagsFor(mode))).toBe(mode);
+    }
+  });
+
+  it('never sets more than one independent behaviour at once', () => {
+    for (const mode of MODES) {
+      const f = transitionFlagsFor(mode);
+      // crossfade and gapless are the two independent toggles; trim only rides on crossfade
+      expect(f.gaplessEnabled && f.crossfadeEnabled).toBe(false);
+      if (f.crossfadeTrimSilence) expect(f.crossfadeEnabled).toBe(true);
+    }
+  });
+});
+
+describe('setTransitionMode', () => {
+  beforeEach(() => {
+    setTransitionMode('none');
+  });
+
+  it('applies the flags atomically and enforces exclusivity', () => {
+    setTransitionMode('crossfade');
+    let s = useAuthStore.getState();
+    expect(s.crossfadeEnabled).toBe(true);
+    expect(s.crossfadeTrimSilence).toBe(false);
+    expect(s.gaplessEnabled).toBe(false);
+
+    setTransitionMode('autodj');
+    s = useAuthStore.getState();
+    expect(s.crossfadeEnabled).toBe(true);
+    expect(s.crossfadeTrimSilence).toBe(true);
+    expect(s.gaplessEnabled).toBe(false);
+
+    setTransitionMode('gapless');
+    s = useAuthStore.getState();
+    expect(s.gaplessEnabled).toBe(true);
+    expect(s.crossfadeEnabled).toBe(false);
+    expect(s.crossfadeTrimSilence).toBe(false);
+
+    setTransitionMode('none');
+    s = useAuthStore.getState();
+    expect(s.gaplessEnabled).toBe(false);
+    expect(s.crossfadeEnabled).toBe(false);
+    expect(s.crossfadeTrimSilence).toBe(false);
+  });
+
+  it('preserves crossfadeSecs when switching between crossfade and autodj', () => {
+    useAuthStore.getState().setCrossfadeSecs(7);
+    setTransitionMode('crossfade');
+    setTransitionMode('autodj');
+    setTransitionMode('crossfade');
+    expect(useAuthStore.getState().crossfadeSecs).toBe(7);
+  });
+});