Support internationalized domain names (IDN) in email validation

Add IdnEmail validation rule that converts IDN domains to Punycode
via idn_to_ascii() before validating with FILTER_VALIDATE_EMAIL.
Applied to all email fields: customers, members, profiles, admin
users, customer portal profiles, and mail configuration.

Includes unit tests for standard emails, IDN emails, and invalid
inputs.

Fixes #388

app/Rules/IdnEmail.php

@@ -0,0 +1,46 @@
+<?php
+
+namespace App\Rules;
+
+use Closure;
+use Illuminate\Contracts\Validation\ValidationRule;
+
+/**
+ * Validates email addresses including those with internationalized
+ * domain names (IDN) like user@münchen.de or michel@exempleé.fr.
+ *
+ * Converts the domain part to Punycode (ASCII) before validation.
+ */
+class IdnEmail implements ValidationRule
+{
+    public function validate(string $attribute, mixed $value, Closure $fail): void
+    {
+        if (! is_string($value) || $value === '') {
+            return;
+        }
+
+        $parts = explode('@', $value);
+
+        if (count($parts) !== 2) {
+            $fail('The :attribute must be a valid email address.');
+
+            return;
+        }
+
+        [$local, $domain] = $parts;
+
+        if (function_exists('idn_to_ascii') && $domain !== '') {
+            $ascii = idn_to_ascii($domain, IDNA_DEFAULT, INTL_IDNA_VARIANT_UTS46);
+
+            if ($ascii !== false) {
+                $domain = $ascii;
+            }
+        }
+
+        $normalized = $local.'@'.$domain;
+
+        if (! filter_var($normalized, FILTER_VALIDATE_EMAIL)) {
+            $fail('The :attribute must be a valid email address.');
+        }
+    }
+}