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

backend/Tests/Musify.Application.Tests/Configuration/GetPlaybackPublicConfigTests.cs

@@ -0,0 +1,31 @@
+using Musify.Application.Configuration;
+using Musify.Application.Tests.TestSupport;
+using Xunit;
+
+namespace Musify.Application.Tests.Configuration
+{
+    public sealed class GetPlaybackPublicConfigTests
+    {
+        [Fact]
+        public async Task Handle_ReflectsPlaybackConfiguration()
+        {
+            var playback = TestConfigurations.Playback(allowAnonymousListening: true, anonymousFragmentSeconds: 30);
+            var handler = new GetPlaybackPublicConfigQueryHandler(playback);
+
+            var response = await handler.Handle(new GetPlaybackPublicConfigQuery(), TestContext.Current.CancellationToken);
+
+            Assert.True(response.AllowAnonymousListening);
+            Assert.Equal(30, response.AnonymousFragmentSeconds);
+        }
+
+        [Fact]
+        public async Task Handle_DefaultsToDisabled()
+        {
+            var handler = new GetPlaybackPublicConfigQueryHandler(TestConfigurations.Playback());
+
+            var response = await handler.Handle(new GetPlaybackPublicConfigQuery(), TestContext.Current.CancellationToken);
+
+            Assert.False(response.AllowAnonymousListening);
+        }
+    }
+}