删除资源引用缩略图里已不可达的 ui kind 分支

- ResourceReferenceInput 缩略图：删掉 `kind === 'ui'` 判断与只服务它的 `kind` 变量，UI 设计文档仍由 `mediaType === 'application/json'` 识别。
- 理由：manifest kind 由 Rust 枚举序列化，只会是 canonical 成员或 `unknown`，该分支永远无法命中。

apps/ai-game-creator-shell/src/features/project-workspace/ResourceReferenceInput.tsx

@@ -1265,7 +1265,6 @@ function ResourcePickerThumbnail({
   const [previewUrl, setPreviewUrl] = useState<string | null>(null);
   const [failed, setFailed] = useState(false);
   const mediaType = asset?.mediaType.toLowerCase() ?? '';
-  const kind = asset?.kind.toLowerCase() ?? '';
 
   useEffect(() => {
     setPreviewUrl(null);
@@ -1312,10 +1311,7 @@ function ResourcePickerThumbnail({
   if (mediaType.startsWith('image/')) {
     return <Loader2 size={18} className="animate-spin" aria-hidden="true" />;
   }
-  if (
-    kind === 'ui' ||
-    mediaType === GAME_CREATION_APP_UI_DESIGN_DOC_MEDIA_TYPE
-  ) {
+  if (mediaType === GAME_CREATION_APP_UI_DESIGN_DOC_MEDIA_TYPE) {
     return <FileText size={18} aria-hidden="true" />;
   }
   return <ImageIcon size={18} aria-hidden="true" />;