fix(engine): add modal scrim guard to toggle_stats_screen (#75)

Pressing S (or the Stats HUD button) while another modal was open
(Settings, Profile, Leaderboard, etc.) would spawn a second ModalScrim
on top of the existing one, violating the one-scrim-at-a-time invariant.

Add other_modal_scrims: Query<(), (With<ModalScrim>, Without<StatsScreen>)>
matching the guard pattern used by every other modal-spawning system.
Also import ModalScrim which was previously not imported in this file.

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

solitaire_engine/src/stats_plugin.rs

@@ -29,7 +29,7 @@ use crate::progress_plugin::ProgressResource;
 use crate::resources::GameStateResource;
 use crate::time_attack_plugin::TimeAttackResource;
 use crate::ui_modal::{
-    ButtonVariant, ModalButton, ScrimDismissible, spawn_modal, spawn_modal_actions,
+    ButtonVariant, ModalButton, ModalScrim, ScrimDismissible, spawn_modal, spawn_modal_actions,
     spawn_modal_button, spawn_modal_header,
 };
 use crate::ui_theme::{
@@ -649,6 +649,7 @@ fn toggle_stats_screen(
     latest_replay: Res<ReplayHistoryResource>,
     selected_index: Res<SelectedReplayIndex>,
     screens: Query<Entity, With<StatsScreen>>,
+    other_modal_scrims: Query<(), (With<ModalScrim>, Without<StatsScreen>)>,
 ) {
     let button_clicked = requests.read().count() > 0;
     if !keys.just_pressed(KeyCode::KeyS) && !button_clicked {
@@ -657,6 +658,9 @@ fn toggle_stats_screen(
     if let Ok(entity) = screens.single() {
         commands.entity(entity).despawn();
     } else {
+        if !other_modal_scrims.is_empty() {
+            return;
+        }
         spawn_stats_screen(
             &mut commands,
             &stats.0,