fix: clear old subtitle cues when switching to scene without subtitles

Previously when subtitleUrl became null, the old cues array was retained
and would replay from the beginning on the next scene.

src/components/Subtitles.vue

@@ -17,7 +17,13 @@ const currentText = ref('')
 const loadedUrl = ref('')
 
 watch(() => props.subtitleUrl, async (url) => {
-  if (!url || url === loadedUrl.value) return
+  if (!url) {
+    cues.value = []
+    currentText.value = ''
+    loadedUrl.value = ''
+    return
+  }
+  if (url === loadedUrl.value) return
   loadedUrl.value = url
   try {
     const resp = await fetch(url)