Narrow the stat cards (cap column width, center grid)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/components/datasets/protein/StatsPanel.css

@@ -1,6 +1,7 @@
 .stats-panel {
   display: grid;
-  grid-template-columns: repeat(2, 1fr);
+  grid-template-columns: repeat(2, minmax(0, 280px));
+  justify-content: center;
   gap: 15px;
   padding: 0;
 }