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.Api.Tests/Endpoints/ConfigEndpointsTests.cs

@@ -0,0 +1,25 @@
+using System.Net;
+using System.Net.Http.Json;
+using Musify.Api.Tests.TestSupport;
+using Musify.Application.Configuration.Responses;
+using Xunit;
+
+namespace Musify.Api.Tests.Endpoints
+{
+    [Collection(ApiCollection.Name)]
+    public sealed class ConfigEndpointsTests(ApiTestFixture fixture)
+    {
+        [Fact]
+        public async Task GetPlaybackConfig_WithoutAuthentication_ReturnsConfiguredValues()
+        {
+            var client = fixture.CreateAnonymousClient();
+
+            var response = await client.GetAsync("/config/playback", TestContext.Current.CancellationToken);
+
+            Assert.Equal(HttpStatusCode.OK, response.StatusCode);
+            var body = await response.Content.ReadFromJsonAsync<PlaybackPublicConfigResponse>(TestContext.Current.CancellationToken);
+            Assert.NotNull(body);
+            Assert.False(body.AllowAnonymousListening);
+        }
+    }
+}