Add anonymous listening support and redesign web-player UI

Backend:
- Expose GET /config/playback so the frontend can detect whether
  anonymous listening is enabled without requiring auth
- Wire anonymous access through track streaming and YouTube resolve flows

Frontend:
- Allow browsing/listening without a session when the backend enables it
- Redesign Inicio, Sidebar and top bar with a new dark theme, typography,
  and layout (top bar with live quick-search, sticky/blurred on scroll)
- Populate Top música, Popular and Recomendados with real Musify +
  YouTube Music data instead of placeholders
- Various shape, spacing, color and animation refinements across the
  redesigned components

web-player/src/lib/tracks.ts

@@ -26,6 +26,10 @@ export function targetExplicit(target: TrackTarget): boolean {
 	return target.kind === 'youtube' ? target.song.isExplicit : (target.track.isExplicit ?? false);
 }
 
+export function targetDurationSeconds(target: TrackTarget): number | undefined {
+	return target.kind === 'youtube' ? target.song.durationSeconds : undefined;
+}
+
 export function targetCoverSrc(target: TrackTarget): string | undefined {
 	return target.kind === 'youtube' ? target.song.thumbnailUrl : undefined;
 }