Add likes, follows and playlist visibility; redesign web-player UI

Backend:
- New Likes feature (toggle/get liked tracks) and endpoints
- User follow/unfollow, listening stats, public user profile
- Playlist visibility (public/private) support
- Various auth and DTO/response adjustments to support the above

Frontend (web-player):
- New design system pass: theme tokens/mode (light/dark), glass menu,
  tabs bar, player dock/extras/queue components, mix tiles, artist link
- New routes: Liked, public user profile (/u/[id]), likes API proxy
- Search refactor: shared SearchBox component used by TopBar and
  MobileHeader (search now reachable on mobile), live results as you
  type instead of requiring Enter, dropped the "Buscando…" quick-result
  preview
- Removed now-unused components (MobileNav, CoverBadge, MixBentoTile,
  PlayPauseOverlay, SuggestionsTable, userDirectory)
- Design handoff reference docs added under web-player/design_handoff_musify_player
  and web-player/docs/design-system-refactor-plan.md

backend/Core/Musify.Application/Serialization/LongAsStringConverter.cs

@@ -0,0 +1,44 @@
+using System.Globalization;
+using System.Text.Json;
+using System.Text.Json.Serialization;
+
+namespace Musify.Application.Serialization;
+
+// Zitadel-issued user ids routinely exceed Number.MAX_SAFE_INTEGER (2^53-1).
+// Serialized as a raw JSON number, the frontend's JSON.parse silently rounds
+// them to the nearest representable double, corrupting the id (it becomes a
+// different, sometimes valid, user id). Applied via [property: JsonConverter(...)]
+// on the specific id-shaped `long`/`long?` response properties (not globally —
+// unrelated `long` fields like byte sizes must stay numbers), this serializes
+// them as a JSON string instead, which round-trips exactly.
+public sealed class LongAsStringConverter : JsonConverter<long>
+{
+    public override long Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options) =>
+        reader.TokenType == JsonTokenType.String
+            ? long.Parse(reader.GetString()!, CultureInfo.InvariantCulture)
+            : reader.GetInt64();
+
+    public override void Write(Utf8JsonWriter writer, long value, JsonSerializerOptions options) =>
+        writer.WriteStringValue(value.ToString(CultureInfo.InvariantCulture));
+}
+
+public sealed class NullableLongAsStringConverter : JsonConverter<long?>
+{
+    public override long? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
+    {
+        if (reader.TokenType == JsonTokenType.Null)
+            return null;
+
+        return reader.TokenType == JsonTokenType.String
+            ? string.IsNullOrEmpty(reader.GetString()) ? null : long.Parse(reader.GetString()!, CultureInfo.InvariantCulture)
+            : reader.GetInt64();
+    }
+
+    public override void Write(Utf8JsonWriter writer, long? value, JsonSerializerOptions options)
+    {
+        if (value.HasValue)
+            writer.WriteStringValue(value.Value.ToString(CultureInfo.InvariantCulture));
+        else
+            writer.WriteNullValue();
+    }
+}