feat(categories): replace procedural SVG art with Gemini-generated WebP editorial banners

src/styles/global.css

@@ -799,10 +799,18 @@ a.category-card:hover { border-color: var(--brand); }
   border-bottom: 1px solid var(--border);
   background: #0d1220;
 }
-.category-card .art svg {
+.category-card .art svg,
+.category-card .art img,
+.category-card .art picture {
   width: 100%;
   height: auto;
+  aspect-ratio: 16 / 9;
+  object-fit: cover;
   display: block;
+  transition: transform 0.25s ease;
+}
+a.category-card:hover .art img {
+  transform: scale(1.02);
 }
 .category-card .body {
   padding: 1.05rem 1.2rem 1.2rem;
@@ -856,7 +864,15 @@ a.category-card:hover { border-color: var(--brand); }
   line-height: 0;
   background: #0d1220;
 }
-.cat-art-wrap svg { width: 100%; height: auto; display: block; }
+.cat-art-wrap svg,
+.cat-art-wrap img,
+.cat-art-wrap picture {
+  width: 100%;
+  height: auto;
+  aspect-ratio: 16 / 9;
+  object-fit: cover;
+  display: block;
+}
 .cat-count {
   margin: 1.25rem 0 1.75rem;
   font-family: var(--font-mono);