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

tests/Unit/Rules/IdnEmailTest.php

@@ -0,0 +1,67 @@
+<?php
+
+use App\Rules\IdnEmail;
+use Illuminate\Support\Facades\Validator;
+
+test('accepts standard email addresses', function () {
+    $validator = Validator::make(
+        ['email' => 'user@example.com'],
+        ['email' => [new IdnEmail]]
+    );
+
+    expect($validator->passes())->toBeTrue();
+});
+
+test('accepts IDN email with accented domain', function () {
+    $validator = Validator::make(
+        ['email' => 'michel@exempleé.fr'],
+        ['email' => [new IdnEmail]]
+    );
+
+    expect($validator->passes())->toBeTrue();
+});
+
+test('accepts IDN email with umlaut domain', function () {
+    $validator = Validator::make(
+        ['email' => 'user@münchen.de'],
+        ['email' => [new IdnEmail]]
+    );
+
+    expect($validator->passes())->toBeTrue();
+});
+
+test('rejects invalid email without at sign', function () {
+    $validator = Validator::make(
+        ['email' => 'notanemail'],
+        ['email' => [new IdnEmail]]
+    );
+
+    expect($validator->passes())->toBeFalse();
+});
+
+test('rejects email with multiple at signs', function () {
+    $validator = Validator::make(
+        ['email' => 'user@@example.com'],
+        ['email' => [new IdnEmail]]
+    );
+
+    expect($validator->passes())->toBeFalse();
+});
+
+test('accepts empty string without failing', function () {
+    $validator = Validator::make(
+        ['email' => ''],
+        ['email' => ['nullable', new IdnEmail]]
+    );
+
+    expect($validator->passes())->toBeTrue();
+});
+
+test('accepts null without failing', function () {
+    $validator = Validator::make(
+        ['email' => null],
+        ['email' => ['nullable', new IdnEmail]]
+    );
+
+    expect($validator->passes())->toBeTrue();
+});