Cards: tighter media on phones (single-column)

On phones the feed is a single column, so the banner no longer needs to keep a
uniform grid height. Below 540px: trim photo banners (16/9 → 2/1) and shrink
image-less placeholders to a slim 54px topic-label band. Desktop/2-column
unchanged (uniform heights still matter there).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/lib/components/ArticleCard.svelte

@@ -373,4 +373,13 @@
     .hero h3, .herotype h3 { font-size: 1.6rem; }
     .herotype .body { padding: 30px 24px; }
   }
+
+  /* On phones the feed is a single column, so the banner no longer carries the
+     job of keeping a uniform grid height. Trim photos to reclaim vertical
+     space, and shrink image-less placeholders to a slim topic label band. */
+  @media (max-width: 540px) {
+    .tile .media { aspect-ratio: 2 / 1; }
+    .tile .media.placeholder { aspect-ratio: auto; height: 54px; }
+    .tile .media.placeholder .ph-word { font-size: 1.15rem; }
+  }
 </style>