Add web layout components

src/view/com/util/Views.tsx

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+export {View as CenteredView, FlatList, ScrollView} from 'react-native'

src/view/com/util/Views.web.tsx

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+/**
+ * In the Web build, we center all content so that it mirrors the
+ * mobile experience (a single narrow column). We then place a UI
+ * shell around the content if you're in desktop.
+ *
+ * Because scrolling is handled by components deep in the hierarchy,
+ * we can't just wrap the top-level element with a max width. The
+ * centering has to be done at the ScrollView.
+ *
+ * These components wrap the RN ScrollView-based components to provide
+ * consistent layout. It also provides <CenteredView> for views that
+ * need to match layout but which aren't scrolled.
+ */
+
+import React from 'react'
+import {
+  FlatList as RNFlatList,
+  FlatListProps,
+  ScrollView as RNScrollView,
+  ScrollViewProps,
+  StyleSheet,
+  StyleProp,
+  View,
+  ViewProps,
+} from 'react-native'
+
+export function CenteredView({
+  style,
+  ...props
+}: React.PropsWithChildren<ViewProps>) {
+  style = addStyle(style, styles.container)
+  return <View style={style} {...props} />
+}
+
+export function FlatList<ItemT>({
+  contentContainerStyle,
+  ...props
+}: React.PropsWithChildren<FlatListProps<ItemT>>) {
+  contentContainerStyle = addStyle(contentContainerStyle, styles.container)
+  return <RNFlatList contentContainerStyle={contentContainerStyle} {...props} />
+}
+
+export function ScrollView({
+  contentContainerStyle,
+  ...props
+}: React.PropsWithChildren<ScrollViewProps>) {
+  contentContainerStyle = addStyle(contentContainerStyle, styles.container)
+  return (
+    <RNScrollView contentContainerStyle={contentContainerStyle} {...props} />
+  )
+}
+
+function addStyle<T>(
+  base: StyleProp<T>,
+  addedStyle: StyleProp<T>,
+): StyleProp<T> {
+  if (Array.isArray(base)) {
+    return base.concat([addedStyle])
+  }
+  return [base, addedStyle]
+}
+
+const styles = StyleSheet.create({
+  container: {
+    width: '100%',
+    maxWidth: 600,
+    marginLeft: 'auto',
+    marginRight: 'auto',
+  },
+})