Add app.bsky.embed.gallery embed type support

Wires the new gallery embed (atproto PR #4827) through the display and
compose pipelines while the lexicon is unpublished. Gallery posts render
via the existing carousel for >4 items and grid for <=4. Compose now
allows up to 10 images and auto-promotes images -> gallery above 4
(demotes back when count drops to <=4 so old clients still see legacy
embeds when possible). Drafts persist as the new `embedGallery` field.

Local shim types in `src/lib/api/gallery-embed-shim*.ts` mirror the
lexicon shape; delete both files once @atproto/api ships the generated
types.

src/types/bsky/post.ts

@@ -10,6 +10,8 @@ import {
   AppBskyLabelerDefs,
 } from '@atproto/api'
 
+import {AppBskyEmbedGallery} from '#/lib/api/gallery-embed-shim'
+
 export type Embed =
   | {
       type: 'post'
@@ -47,6 +49,10 @@ export type Embed =
       type: 'images'
       view: $Typed<AppBskyEmbedImages.View>
     }
+  | {
+      type: 'gallery'
+      view: $Typed<AppBskyEmbedGallery.View>
+    }
   | {
       type: 'link'
       view: $Typed<AppBskyEmbedExternal.View>
@@ -122,6 +128,11 @@ export function parseEmbed(embed: AppBskyFeedDefs.PostView['embed']): Embed {
       type: 'images',
       view: embed,
     }
+  } else if (AppBskyEmbedGallery.isView(embed)) {
+    return {
+      type: 'gallery',
+      view: embed,
+    }
   } else if (AppBskyEmbedExternal.isView(embed)) {
     return {
       type: 'link',