feat(mui): PageEnter kit primitive (consistent staggered page entrance) + adopt in WarehouseSuppliers

Each top-level child rises+fades in, staggered (0.4s, ease-out), so every page enters the same way and nothing appears instantly; honors prefers-reduced-motion. Replaces ad-hoc per-page motion.div entrance wrappers. One component → the look is tunable in one place.

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

src/admin/ui/index.ts

@@ -28,3 +28,4 @@ export { default as FileUpload } from "./FileUpload";
 export { default as ProgressBar } from "./ProgressBar";
 export type { ProgressColor } from "./ProgressBar";
 export { default as ThemeToggle } from "./ThemeToggle";
+export { default as PageEnter } from "./PageEnter";