Split diff horizontal scroll into platform-specific components

Use Metro .web.tsx resolution to separate native gesture arbitration
(RNGH ScrollView, waitFor, closeGestureRef) from web (plain RN ScrollView),
fixing text selection drag on desktop web.

packages/app/src/components/diff-scroll.web.tsx

@@ -0,0 +1,29 @@
+import { ScrollView, type LayoutChangeEvent, type StyleProp, type ViewStyle } from "react-native";
+
+interface DiffScrollProps {
+  children: React.ReactNode;
+  scrollViewWidth: number;
+  onScrollViewWidthChange: (width: number) => void;
+  style?: StyleProp<ViewStyle>;
+  contentContainerStyle?: StyleProp<ViewStyle>;
+}
+
+export function DiffScroll({
+  children,
+  onScrollViewWidthChange,
+  style,
+  contentContainerStyle,
+}: DiffScrollProps) {
+  return (
+    <ScrollView
+      horizontal
+      nestedScrollEnabled
+      showsHorizontalScrollIndicator
+      style={style}
+      contentContainerStyle={contentContainerStyle}
+      onLayout={(e: LayoutChangeEvent) => onScrollViewWidthChange(e.nativeEvent.layout.width)}
+    >
+      {children}
+    </ScrollView>
+  );
+}