Prevent horizontal page scroll on mobile

Clamp html/body to viewport width with overflow-x hidden; wide content like the
data table keeps its own internal horizontal scroll.

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

frontend/src/index.css

@@ -3015,3 +3015,16 @@ html {
 .atomic-scroll-area::-webkit-scrollbar-thumb:hover {
   background-color: #64748b;
 }
+
+/* Prevent horizontal page scroll (esp. on mobile): clamp to viewport width.
+   Wide content (e.g. the data table) scrolls inside its own overflow:auto wrapper,
+   so this only stops the whole window from sliding left/right. */
+html,
+body {
+  max-width: 100%;
+  overflow-x: hidden;
+}
+
+#root {
+  overflow-x: clip;
+}