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/SectionHeading.svelte

@@ -11,14 +11,16 @@
 </script>
 
 {#if actions}
-	<div class="mb-3 flex items-baseline justify-between gap-4 sm:mb-4 {klass}">
-		<h2 class="font-display font-medium tracking-tight text-fg sm:text-xl">
+	<div class="mb-4 flex items-end justify-between gap-4 sm:mb-5 {klass}">
+		<h2 class="font-display text-xl font-medium tracking-tight text-fg sm:text-[1.75rem]">
 			{title}
 		</h2>
 		{@render actions()}
 	</div>
 {:else}
-	<h2 class="mb-3 font-display font-medium tracking-tight text-fg sm:mb-4 sm:text-xl {klass}">
+	<h2
+		class="mb-4 font-display text-xl font-medium tracking-tight text-fg sm:mb-5 sm:text-[1.75rem] {klass}"
+	>
 		{title}
 	</h2>
 {/if}