Fix scroll-to-top when selecting language on course introduction

Only run scroll in course/stage route when the transition is to a
course.stage route. Query-param-only changes (e.g. repo/track after
language selection) no longer trigger scroll, so the page stays in
place when choosing a language.

Author: VasylMarchuk

app/routes/course/stage.ts

@@ -1,6 +1,7 @@
 import BaseRoute from 'codecrafters-frontend/utils/base-route';
 import scrollToTop from 'codecrafters-frontend/utils/scroll-to-top';
 import { service } from '@ember/service';
+import type Transition from '@ember/routing/transition';
 import type RouterService from '@ember/routing/router-service';
 import type CoursePageStateService from 'codecrafters-frontend/services/course-page-state';
 import type { ModelType as CourseRouteModelType } from 'codecrafters-frontend/routes/course';
@@ -17,7 +18,14 @@ export default class CourseStageRoute extends BaseRoute {
   constructor(...args: object[]) {
     super(...args);
 
-    this.router.on('routeDidChange', () => {
+    this.router.on('routeDidChange', (transition: Transition) => {
+      const toRouteName = transition?.to?.name;
+      const isNavigatingToStage = toRouteName && String(toRouteName).startsWith('course.stage');
+
+      if (!isNavigatingToStage) {
+        return;
+      }
+
       const element = document.querySelector('#course-page-scrollable-area');
 
       if (element) {