don't use flexBasis on web for message post embeds (#4303)

* don't use flexBasis on web

* rm unnecessary style

src/components/dms/MessageItemEmbed.tsx

@@ -3,7 +3,7 @@ import {View} from 'react-native'
 import {AppBskyEmbedRecord} from '@atproto/api'
 
 import {PostEmbeds} from '#/view/com/util/post-embeds'
-import {atoms as a, useTheme} from '#/alf'
+import {atoms as a, native, useTheme} from '#/alf'
 
 let MessageItemEmbed = ({
   embed,
@@ -13,7 +13,7 @@ let MessageItemEmbed = ({
   const t = useTheme()
 
   return (
-    <View style={[a.my_xs, t.atoms.bg, a.rounded_md, {flexBasis: 0}]}>
+    <View style={[a.my_xs, t.atoms.bg, native({flexBasis: 0})]}>
       <PostEmbeds embed={embed} />
     </View>
   )