feat(mui): add FileUpload kit primitive (hidden input + selected-file chips)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/admin/ui/index.ts

@@ -23,3 +23,4 @@ export { default as LoadingState } from "./LoadingState";
 export { default as FilterBar } from "./FilterBar";
 export { default as StatCard } from "./StatCard";
 export type { StatCardColor } from "./StatCard";
+export { default as FileUpload } from "./FileUpload";