fix(theme): clip overflow-x at root, not body

A scroll-container body makes Firefox clip descendant composited
transforms that briefly overshoot the viewport.

src/shared/styles/theme.css

@@ -166,6 +166,9 @@
     /* Reserve scrollbar gutter so locking body scroll (e.g. when a modal
      * opens) doesn't widen the viewport and shift fixed elements. */
     scrollbar-gutter: stable;
+    /* Clip at the root, not body: a scroll-container body makes Firefox clip
+     * descendant composited transforms that briefly overshoot the viewport. */
+    overflow-x: hidden;
   }
 
   body {
@@ -173,7 +176,6 @@
     font-family: var(--font-body);
     font-weight: var(--font-weight-body);
     line-height: var(--line-height-tight);
-    overflow-x: hidden;
   }
 
   /* Page-wide blue dot grain. z-index -1 drops it behind body's content so