fix article read after switching to article view

vue/src/composables/useFeeds.js

@@ -267,6 +267,13 @@ function toggleViewMode() {
   if (viewMode.value === 'article') {
     leaveArticleView()
   } else {
+    // Disconnect first: the v-if switch is about to unmount all .observe
+    // elements, which would otherwise fire intersection callbacks reporting
+    // them as no-longer-intersecting and mark every visible article read.
+    if (observer) {
+      observer.disconnect()
+      observer = null
+    }
     viewMode.value = 'article'
     currentIndex.value = 0
     markCurrentArticleRead()