Clear previous LastLocalUserScore when returning to song select

This seems like the lowest friction way of fixing
ppy#30885.

We could also only null this on application, but this feels worse
because

- It would require local handling (potentially complex) in
  `BeatmapOffsetControl` if we want to continue displaying the graph and
button after clicking it.
- It would make the session static very specific in usage and
  potentially make future usage not possible due to being nulled in only
a very specific scenario.

One might argue that it would be nice to have this non-null until the
next play, but if such a usage comes up I'd propose we rename this
session static and add a new one with that purpose.

osu.Game/Configuration/SessionStatics.cs

@@ -10,6 +10,7 @@
 using osu.Game.Overlays;
 using osu.Game.Overlays.Mods;
 using osu.Game.Scoring;
+using osu.Game.Screens.Play;
 
 namespace osu.Game.Configuration
 {
@@ -77,7 +78,8 @@ public enum Static
         TouchInputActive,
 
         /// <summary>
-        /// Stores the local user's last score (can be completed or aborted).
+        /// Contains the local user's last score (can be completed or aborted) after exiting <see cref="Player"/>.
+        /// Will be cleared to <c>null</c> when leaving <see cref="PlayerLoader"/>.
         /// </summary>
         LastLocalUserScore,
 

osu.Game/Screens/Play/PlayerLoader.cs

@@ -28,6 +28,7 @@
 using osu.Game.Overlays;
 using osu.Game.Overlays.Notifications;
 using osu.Game.Performance;
+using osu.Game.Scoring;
 using osu.Game.Screens.Menu;
 using osu.Game.Screens.Play.PlayerSettings;
 using osu.Game.Skinning;
@@ -78,6 +79,8 @@ public partial class PlayerLoader : ScreenWithBeatmapBackground
         private FillFlowContainer disclaimers = null!;
         private OsuScrollContainer settingsScroll = null!;
 
+        private Bindable<ScoreInfo?> lastScore = null!;
+
         private Bindable<bool> showStoryboards = null!;
 
         private bool backgroundBrightnessReduction;
@@ -179,6 +182,8 @@ private void load(SessionStatics sessionStatics, OsuConfigManager config)
         {
             muteWarningShownOnce = sessionStatics.GetBindable<bool>(Static.MutedAudioNotificationShownOnce);
             batteryWarningShownOnce = sessionStatics.GetBindable<bool>(Static.LowBatteryNotificationShownOnce);
+            lastScore = sessionStatics.GetBindable<ScoreInfo?>(Static.LastLocalUserScore);
+
             showStoryboards = config.GetBindable<bool>(OsuSetting.ShowStoryboard);
 
             const float padding = 25;
@@ -347,6 +352,8 @@ public override bool OnExiting(ScreenExitEvent e)
             highPerformanceSession?.Dispose();
             highPerformanceSession = null;
 
+            lastScore.Value = null;
+
             return base.OnExiting(e);
         }