feat: v1.8.0 — new themes, queue toolbar, lightbox, i18n (fr/nl)

Themes:
- Add Poison (phosphor green on charcoal), Nucleo (warm brass light), Classic Winamp (LCD glow, Winamp yellow/orange) themes
- Overhaul Psychowave — refined deep violet, no longer WIP
- Reorganise ThemePicker into groups: Catppuccin, Nord, Retro, Tokyo Night, Psysonic Themes
- Add --volume-accent CSS var for per-theme volume slider colour override

Queue:
- Full toolbar redesign with centred round buttons (Shuffle/Save/Load/Clear/Gapless/Crossfade)
- Crossfade popover with 1–10 s range slider, right-aligned to avoid viewport overflow
- Queue header: title + count + duration inline in accent colour, close button removed
- Tech info (codec/bitrate) as frosted-glass overlay badge on cover art

UI:
- CoverLightbox shared component for album cover (AlbumHeader) and artist avatar (ArtistDetail)
- NowPlayingDropdown: separate spinning/loading state — button always clickable, icon spins 600 ms min
- Settings: "Experimental" badge on Crossfade and Gapless toggles
- Help page: 2-column grid layout, new Crossfade & Gapless Q&A entry, updated theme/language entries

i18n:
- Full French (fr) and Dutch (nl) translations across all namespaces
- Language selector sorted alphabetically (nl, en, fr, de)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/pages/Help.tsx

@@ -45,6 +45,7 @@ export default function Help() {
         { q: t('help.q22'), a: t('help.a22') },
         { q: t('help.q23'), a: t('help.a23') },
         { q: t('help.q24'), a: t('help.a24') },
+        { q: t('help.q29'), a: t('help.a29') },
       ],
     },
     {
@@ -100,7 +101,7 @@ export default function Help() {
     <div className="content-body animate-fade-in">
       <h1 className="page-title" style={{ marginBottom: '2rem' }}>{t('help.title')}</h1>
 
-      <div style={{ display: 'flex', flexDirection: 'column', gap: '2rem' }}>
+      <div style={{ display: 'grid', gridTemplateColumns: 'repeat(2, 1fr)', gap: '1.25rem', alignItems: 'start' }}>
         {sections.map((section, si) => (
           <section key={si} className="settings-section">
             <div className="settings-section-header">