Send mods in spectator frame headers

osu.Game/Online/Spectator/FrameHeader.cs

@@ -3,8 +3,10 @@
 
 using System;
 using System.Collections.Generic;
+using System.Linq;
 using MessagePack;
 using Newtonsoft.Json;
+using osu.Game.Online.API;
 using osu.Game.Rulesets.Scoring;
 using osu.Game.Scoring;
 
@@ -56,6 +58,17 @@ public class FrameHeader
         [Key(6)]
         public DateTimeOffset ReceivedTime { get; set; }
 
+        /// <summary>
+        /// The set of mods currently active.
+        /// </summary>
+        /// <remarks>
+        /// Nullable for backwards compatibility with older clients
+        /// (these structures are also used server-side, and <see langword="null"/> will be used as marker that the data isn't there).
+        /// can be made non-nullable 20250407
+        /// </remarks>
+        [Key(7)]
+        public APIMod[]? Mods { get; set; }
+
         /// <summary>
         /// Construct header summary information from a point-in-time reference to a score which is actively being played.
         /// </summary>
@@ -69,6 +82,7 @@ public FrameHeader(ScoreInfo score, ScoreProcessorStatistics statistics)
             MaxCombo = score.MaxCombo;
             // copy for safety
             Statistics = new Dictionary<HitResult, int>(score.Statistics);
+            Mods = score.APIMods.ToArray();
 
             ScoreProcessorStatistics = statistics;
         }