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/store/themeStore.ts

@@ -1,7 +1,7 @@
 import { create } from 'zustand';
 import { persist } from 'zustand/middleware';
 
-type Theme = 'mocha' | 'macchiato' | 'frappe' | 'latte' | 'nord' | 'nord-snowstorm' | 'nord-frost' | 'nord-aurora' | 'psychowave';
+type Theme = 'mocha' | 'macchiato' | 'frappe' | 'latte' | 'nord' | 'nord-snowstorm' | 'nord-frost' | 'nord-aurora' | 'psychowave' | 'classic-winamp' | 'poison' | 'nucleo' | 'gruvbox-dark-hard' | 'gruvbox-dark-medium' | 'gruvbox-dark-soft' | 'gruvbox-light-hard' | 'gruvbox-light-medium' | 'gruvbox-light-soft' | 'tokyo-night' | 'tokyo-night-storm' | 'tokyo-night-light';
 
 interface ThemeState {
   theme: Theme;