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/api/lastfm.ts

@@ -1,4 +1,5 @@
 import { invoke } from '@tauri-apps/api/core';
+import { useAuthStore } from '../store/authStore';
 
 const API_KEY = '9917fb39049225a13bec225ad6d49054';
 const API_SECRET = '03817dda02bee87a178aab7581abae3b';
@@ -15,13 +16,24 @@ function errMsg(e: unknown): string {
 
 async function call(params: Record<string, string>, sign = false, get = false): Promise<any> {
   const entries = Object.entries(params) as [string, string][];
-  return invoke('lastfm_request', {
-    params: entries,
-    sign,
-    get,
-    apiKey: API_KEY,
-    apiSecret: API_SECRET,
-  });
+  try {
+    const result = await invoke('lastfm_request', {
+      params: entries,
+      sign,
+      get,
+      apiKey: API_KEY,
+      apiSecret: API_SECRET,
+    });
+    // Clear session error on any successful authenticated call
+    if (sign) useAuthStore.getState().setLastfmSessionError(false);
+    return result;
+  } catch (e) {
+    // Last.fm error codes 4, 9, 14 = auth/session invalid
+    if (sign && /^Last\.fm (4|9|14)\b/.test(errMsg(e))) {
+      useAuthStore.getState().setLastfmSessionError(true);
+    }
+    throw e;
+  }
 }
 
 export async function lastfmGetToken(): Promise<string> {