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/components/ui/Button.svelte

@@ -36,7 +36,7 @@
 
 	const variants = {
 		primary: 'bg-accent-soft text-on-accent active:scale-[0.97]',
-		secondary: 'bg-surface-2 text-fg hover:bg-accent/12',
+		secondary: 'bg-bg text-fg ring-1 ring-line hover:bg-surface',
 		subtle: 'bg-white/10 text-fg hover:bg-accent/18',
 		danger: 'bg-danger text-on-accent hover:brightness-110'
 	};