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/Rail.svelte

@@ -7,9 +7,15 @@
 	}
 
 	let { class: klass = '', children }: Props = $props();
+
+	let track: HTMLDivElement | undefined = $state();
+
+	export function scrollByPage(direction: 1 | -1) {
+		track?.scrollBy({ left: direction * track.clientWidth * 0.85, behavior: 'smooth' });
+	}
 </script>
 
-<div class="no-scrollbar -mx-1 flex gap-4 overflow-x-auto px-1 pb-2 {klass}">
+<div bind:this={track} class="no-scrollbar -mx-1 flex gap-4 overflow-x-auto px-1 pb-2 {klass}">
 	{@render children()}
 </div>