Add editable, previewable payment-due email template

The "Payment due" notice the daily billing scan sends was fixed wording
baked into PaymentDueMailer. Studio admins can now view, edit and preview
its subject and body under Studio Settings -> Payment Due Email.

- PaymentDueEmailTemplate stores subject/body/item-line as us_payment_due_email_*
  options, each falling back to the built-in default when blank, and renders
  via {token} substitution.
- PaymentDueMailer now builds a token map from the same consolidated
  items/credit/e-transfer/reference data and renders the stored template; the
  default template reproduces the previous wording exactly.
- PaymentEmailController serves the view/edit/save/reset admin page
  (manage_billing, nonce-checked) with a server-rendered sample preview.
- PaymentEmailPreviewEndpoint (POST us-scheduler/v1/payment-email/preview,
  manage_billing) + payment-email-admin.js drive a live preview of unsaved edits.
- Submenu wired under Studio Settings; endpoint wired in RestRegistrar.
- Bump to 1.6.0 and add CHANGELOG entry. New options default gracefully, so
  existing sites need no migration and send the identical notice until edited.

Co-authored-by: anthropic/claude-opus-4-8

tests/Unit/Payment/PaymentDueMailerTest.php

@@ -5,15 +5,27 @@ namespace Unsupervised\Schedular\Tests\Unit\Payment;
 
 use Brain\Monkey\Functions;
 use Mockery;
+use Unsupervised\Schedular\Payment\PaymentDueEmailTemplate;
 use Unsupervised\Schedular\Payment\PaymentDueMailer;
 use Unsupervised\Schedular\Tests\Unit\TestCase;
 
 class PaymentDueMailerTest extends TestCase
 {
-    private function student(string $email): \WP_User
+    protected function setUp(): void
     {
-        $student             = Mockery::mock(\WP_User::class);
-        $student->user_email = $email;
+        parent::setUp();
+
+        // The mailer renders from PaymentDueEmailTemplate, which reads its
+        // subject/body/item-line from options. An empty stored value means the
+        // built-in default template is used — the behaviour these tests assert.
+        Functions\when('get_option')->alias(static fn (string $name, $default = '') => '');
+    }
+
+    private function student(string $email, string $name = 'Alex Student'): \WP_User
+    {
+        $student               = Mockery::mock(\WP_User::class);
+        $student->user_email   = $email;
+        $student->display_name = $name;
 
         return $student;
     }
@@ -129,4 +141,39 @@ class PaymentDueMailerTest extends TestCase
 
         self::assertTrue((new PaymentDueMailer())->send($this->student('[email protected]'), $items));
     }
+
+    public function testRendersCustomTemplateWithTokens(): void
+    {
+        // A stored template overrides the default; tokens are substituted with
+        // the real values gathered from the items and student.
+        Functions\when('get_option')->alias(static function (string $name) {
+            if ($name === PaymentDueEmailTemplate::OPT_SUBJECT) {
+                return 'Hi {student_name} — {total_due}';
+            }
+            if ($name === PaymentDueEmailTemplate::OPT_BODY) {
+                return "Dear {student_name},\n{items}\nOwing: {total_due}";
+            }
+            if ($name === PaymentDueEmailTemplate::OPT_ITEM_LINE) {
+                return '* {label} = {currency} {amount}';
+            }
+            return '';
+        });
+
+        Functions\expect('wp_mail')
+            ->once()
+            ->with(
+                '[email protected]',
+                'Hi Jordan — CAD 35.00',
+                Mockery::on(static function (string $body): bool {
+                    return str_contains($body, 'Dear Jordan,')
+                        && str_contains($body, '* Piano = CAD 35.00')
+                        && str_contains($body, 'Owing: CAD 35.00');
+                })
+            )
+            ->andReturn(true);
+
+        $items = [[ 'label' => 'Piano', 'amount' => 35.0, 'currency' => 'CAD', 'due_date' => '2026-07-15', 'etransfer_email' => null ]];
+
+        self::assertTrue((new PaymentDueMailer())->send($this->student('[email protected]', 'Jordan'), $items));
+    }
 }