Fix mobile header overlap and thumbnail crop ratio

The language switch overlapped the centered logo on narrow viewports
(~390px), and article card covers used an overly aggressive 4:5 crop
that sliced through text baked into cover images.

frontend/app/components/ArticleCard.vue

@@ -53,7 +53,7 @@ const href = computed(() => localePath(props.locale, `/blog/${props.article.slug
 
 .cover {
   width: 100%;
-  aspect-ratio: 4 / 5;
+  aspect-ratio: 3 / 2;
   object-fit: cover;
   background: var(--color-surface);
   transition: transform 0.4s ease;