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/Http/Requests/AdminUserUpdateRequest.php

@@ -2,6 +2,7 @@
 
 namespace App\Http\Requests;
 
+use App\Rules\IdnEmail;
 use Illuminate\Foundation\Http\FormRequest;
 use Illuminate\Validation\Rule;
 
@@ -18,7 +19,7 @@ class AdminUserUpdateRequest extends FormRequest
             'name' => ['required', 'string'],
             'email' => [
                 'required',
-                'email',
+                new IdnEmail,
                 Rule::unique('users')->ignore($this->route('user')),
             ],
             'phone' => ['nullable', 'string'],