Drop carousel naming, cap gallery MediaPreview to 4 tiles

Renames `GRID_TO_CAROUSEL_THRESHOLD` -> `MAX_GRID_IMAGES` and
`carouselEnabled` -> `useExpandedLayout` in ImageEmbed so the render-path
decision reads as a count threshold rather than naming the component
that happens to live downstream today.

Also slices the gallery branch of MediaPreview to the first 4 items so a
10-image gallery doesn't overflow the inline notification/DM row.

src/components/MediaPreview.tsx

@@ -48,9 +48,11 @@ export function Embed({
       </Outer>
     )
   } else if (e.type === 'gallery') {
+    // Notification/DM preview is a narrow inline strip; cap at 4 tiles so
+    // a 10-image gallery doesn't blow out the row width.
     return (
       <Outer style={style}>
-        {e.view.items.map(item => {
+        {e.view.items.slice(0, 4).map(item => {
           const image: AppBskyEmbedImages.ViewImage = {
             thumb: item.thumbnail,
             fullsize: item.fullsize,

src/components/Post/Embed/ImageEmbed.tsx

@@ -16,7 +16,7 @@ import {useAnalytics} from '#/analytics'
 import {type EmbedType} from '#/types/bsky/post'
 import {type CommonProps} from './types'
 
-const GRID_TO_CAROUSEL_THRESHOLD = 4
+const MAX_GRID_IMAGES = 4
 
 export function ImageEmbed({
   embed,
@@ -35,9 +35,9 @@ export function ImageEmbed({
           aspectRatio: item.aspectRatio,
         }))
       : embed.view.images
-  const carouselEnabled =
+  const useExpandedLayout =
     embed.type === 'gallery'
-      ? images.length > GRID_TO_CAROUSEL_THRESHOLD
+      ? images.length > MAX_GRID_IMAGES
       : ax.features.enabled(ax.features.PostGalleryEmbedEnable)
 
   // Captured from AutoSizedImage so the peek-commit handler can reuse the same
@@ -127,7 +127,7 @@ export function ImageEmbed({
       )
     }
 
-    if (carouselEnabled) {
+    if (useExpandedLayout) {
       return (
         <View style={[a.mt_sm, rest.style]}>
           <Gallery