Initial frontend import

src/adminRouterMode.tsx

@@ -0,0 +1,12 @@
+import { createContext, useContext } from "react";
+
+/** `basename`: admin-only app under `BrowserRouter basename={adminUiPrefix}`. `absolute`: full site `App`. */
+export type AdminRouterMode = "basename" | "absolute";
+
+const AdminRouterModeCtx = createContext<AdminRouterMode>("absolute");
+
+export const AdminRouterModeProvider = AdminRouterModeCtx.Provider;
+
+export function useAdminRouterMode(): AdminRouterMode {
+  return useContext(AdminRouterModeCtx);
+}