feat(titlebar): make custom title bar optional via Settings toggle

- New authStore field useCustomTitlebar (default: true)
- set_window_decorations Tauri command toggles native decorations at runtime
- Settings toggle visible only on Linux (no restart required)
- i18n keys added to EN + DE

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

src/pages/Home.tsx

@@ -134,6 +134,7 @@ export default function Home() {
             {isVisible('mostPlayed') && (
               <AlbumRow
                 title={t('home.mostPlayed')}
+                titleLink="/most-played"
                 albums={mostPlayed}
                 onLoadMore={() => loadMore('frequent', mostPlayed, setMostPlayed)}
                 moreText={t('home.loadMore')}