[APP-2160] Extract StandardSiteEmbed type guards into utils.ts

Move isStandardSiteEmbed alongside a new isStandardSitePublicationEmbed
helper. The publication check now requires at least one publication ref
and zero document refs, replacing a tautological condition that also
returned true for empty associatedRefs arrays.

src/components/Post/Embed/StandardSiteEmbed/utils.ts

@@ -0,0 +1,20 @@
+import {type AppBskyEmbedExternal, AtUri} from '@atproto/api'
+
+export function isStandardSiteEmbed(view: AppBskyEmbedExternal.ViewExternal) {
+  return view.associatedRefs?.some(ref =>
+    new AtUri(ref.uri).collection.startsWith('site.standard.'),
+  )
+}
+
+export function isStandardSitePublicationEmbed(
+  view: AppBskyEmbedExternal.ViewExternal,
+) {
+  return (
+    view.associatedRefs?.some(
+      ref => new AtUri(ref.uri).collection === 'site.standard.publication',
+    ) &&
+    view.associatedRefs.every(
+      ref => new AtUri(ref.uri).collection !== 'site.standard.document',
+    )
+  )
+}