fix: center app screenshots on mobile

styles.css

@@ -405,6 +405,15 @@ textarea {
     grid-template-columns: repeat(3, 1fr);
 }
 
+.mockup {
+    display: flex;
+    justify-content: center;
+}
+
+.mockup-frame {
+    margin-inline: auto;
+}
+
 .mockup-screen ul {
     padding: 0;
     margin-top: 16px;
@@ -1092,6 +1101,10 @@ textarea {
         grid-template-columns: 1fr;
     }
 
+    .screens-grid {
+        justify-items: center;
+    }
+
     .hero-copy h1 {
         font-size: 42px;
         max-width: none;