fix(engine): close Settings, Help, Leaderboard, and theme store on Esc

Phase C dismissal audit: Esc / scrim-tap / Done must behave the same
on every modal. Stragglers found and fixed — none of these had any
Esc path (pause's toggle guard swallowed the key while they were
open):

- Settings: Esc clears SettingsScreen, gated on being the topmost
  modal so a stacked sync-setup / theme-store dialog owns Esc
- Help: Esc closes alongside F1/Done (the code comment already
  claimed an Esc path existed — now it does)
- Leaderboard: Esc closes when topmost; the display-name dialog
  stacked above it now Esc-cancels like sync-setup's dialog
- Theme store: Esc closes (always topmost when open)

Scrim-tap opt-ins are unchanged — ui_modal documents which modals
deliberately stay non-dismissible on outside clicks.

Tests: escape_closes_help_screen, escape_closes_settings_screen_flag.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

solitaire_engine/src/help_plugin.rs

@@ -86,13 +86,18 @@ fn toggle_help_screen(
 }
 
 /// Click handler for the modal's "Done" button. F1 toggles the overlay
-/// the same way; this just exposes the close action to mouse / touch.
+/// the same way; Esc closes too, so dismissal matches every other
+/// modal (Phase C dismissal audit). Nothing ever stacks above Help,
+/// so Esc needs no topmost gate.
 fn handle_help_close_button(
     mut commands: Commands,
+    keys: Res<ButtonInput<KeyCode>>,
     close_buttons: Query<&Interaction, (With<HelpCloseButton>, Changed<Interaction>)>,
     screens: Query<Entity, With<HelpScreen>>,
 ) {
-    if !close_buttons.iter().any(|i| *i == Interaction::Pressed) {
+    let clicked = close_buttons.iter().any(|i| *i == Interaction::Pressed);
+    let esc = keys.just_pressed(KeyCode::Escape) && !screens.is_empty();
+    if !clicked && !esc {
         return;
     }
     for entity in &screens {
@@ -583,4 +588,33 @@ mod tests {
             0
         );
     }
+
+    /// Esc must dismiss the Help modal like Done and F1 do (Phase C
+    /// dismissal audit).
+    #[test]
+    fn escape_closes_help_screen() {
+        let mut app = headless_app();
+        app.world_mut()
+            .resource_mut::<ButtonInput<KeyCode>>()
+            .press(KeyCode::F1);
+        app.update();
+
+        {
+            let mut input = app.world_mut().resource_mut::<ButtonInput<KeyCode>>();
+            input.release(KeyCode::F1);
+            input.clear();
+            input.press(KeyCode::Escape);
+        }
+        app.update();
+        app.update();
+
+        assert_eq!(
+            app.world_mut()
+                .query::<&HelpScreen>()
+                .iter(app.world())
+                .count(),
+            0,
+            "Esc must close the Help modal"
+        );
+    }
 }