Conditionally render ScoreCube to properly clean up WebGL canvas

Only mount the ScoreCube component when on the save-editor page.
This ensures onDestroy is called when navigating away, properly
disposing of the WebGL renderer and removing the canvas from DOM.

src/lib/save-editor/MissionScoresEditor.svelte

@@ -1,4 +1,5 @@
 <script>
+    import { currentPage } from '../../stores.js';
     import ScoreCube from './ScoreCube.svelte';
 
     export let slot;
@@ -13,10 +14,12 @@
 </script>
 
 <div class="scores-editor">
+    {#if $currentPage === 'save-editor'}
         <ScoreCube
             missions={missionData}
             onUpdate={handleCubeUpdate}
         />
+    {/if}
 </div>
 
 <style>