Add width and height attributes to cover images (#519)

- Ensures that the browser can allocate the correct amount of space in the document while the image is loading.
- Reduces CLS #499

assets/css/common/post-entry.css

@@ -98,6 +98,7 @@
     border-radius: var(--radius);
     pointer-events: none;
     width: 100%;
+    height: auto;
 }
 
 .entry-cover a {