Add customer management

internal_frontend/lib/ui/showError.ts

@@ -0,0 +1,16 @@
+// lib/ui/showError.ts
+import {toast} from "sonner";
+
+export function showError(error: unknown, fallback = "Ein unbekannter Fehler ist aufgetreten") {
+    let message: string;
+
+    if (error instanceof Error) {
+        message = error.message;
+    } else if (typeof error === "string") {
+        message = error;
+    } else {
+        message = fallback;
+    }
+
+    toast.error(message);
+}