fix(engine): add missing modal scrim guard to leaderboard panel

toggle_leaderboard_screen was missing the other_modal_scrims guard that
all other panel-toggle systems have. Pressing L (or the HUD button) while
any other modal was open would spawn a second ModalScrim on top of the
existing one, breaking z-ordering and leaving the first modal un-dismissable.

Adds:
  other_modal_scrims: Query<(), (With<ModalScrim>, Without<LeaderboardScreen>)>
and the early-return guard before spawn_leaderboard_screen is called.

Closes #77

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

solitaire_engine/src/leaderboard_plugin.rs

@@ -191,6 +191,7 @@ fn toggle_leaderboard_screen(
     keys: Res<ButtonInput<KeyCode>>,
     mut requests: MessageReader<ToggleLeaderboardRequestEvent>,
     screens: Query<Entity, With<LeaderboardScreen>>,
+    other_modal_scrims: Query<(), (With<ModalScrim>, Without<LeaderboardScreen>)>,
     data: Res<LeaderboardResource>,
     provider: Option<Res<SyncProviderResource>>,
     settings: Option<Res<SettingsResource>>,
@@ -208,6 +209,11 @@ fn toggle_leaderboard_screen(
         return;
     }
 
+    // Don't stack a second modal scrim over one that is already open.
+    if !other_modal_scrims.is_empty() {
+        return;
+    }
+
     // Spawn the panel immediately with whatever data we have so far.
     let remote_available = provider
         .as_ref()