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

src/AdminMenu.php

@@ -32,6 +32,7 @@ use Unsupervised\Schedular\Offering\OfferingRepository;
 use Unsupervised\Schedular\Payment\BillingMethodResolver;
 use Unsupervised\Schedular\Payment\CreditRepository;
 use Unsupervised\Schedular\Payment\PaymentController;
+use Unsupervised\Schedular\Payment\PaymentEmailController;
 use Unsupervised\Schedular\Payment\PaymentReportController;
 use Unsupervised\Schedular\Payment\PaymentRepository;
 use Unsupervised\Schedular\Payment\PaymentService;
@@ -56,6 +57,12 @@ class AdminMenu {
 	 */
 	private string $availabilityHook = '';
 
+	/**
+	 * Hook suffix of the payment-email screen, captured when the page is added so
+	 * its live-preview script loads on that screen only.
+	 */
+	private string $paymentEmailHook = '';
+
 	private AvailabilityController $availabilityController;
 	private LessonController $lessonController;
 	private OfferingController $offeringController;
@@ -69,6 +76,7 @@ class AdminMenu {
 	private StudioSettings $settings;
 	private AccessSettings $accessSettings;
 	private PaymentController $paymentController;
+	private PaymentEmailController $paymentEmailController;
 	private PaymentReportController $paymentReportController;
 
 	public function __construct( AvailabilityRepository $availability, BookingRepository $bookings, OfferingRepository $offerings, QuestionRepository $questions, AnswerRepository $answers, PolicyRepository $policies, PolicyVersionRepository $policyVersions, PolicyService $policyService, AcceptanceRepository $acceptances, InviteRepository $invites, EnrollmentRepository $enrollments, GroupAccessRepository $groupAccess, StudioSettings $settings, PaymentRepository $payments, PaymentService $paymentService, BillingMethodResolver $resolver, RegistrationMailer $registrationMailer, CreditRepository $credits, GuardianService $guardians, LessonBooker $booker, RegistrationGate $gate, BillingModeReconciler $billingModeReconciler ) {
@@ -90,6 +98,7 @@ class AdminMenu {
 		$this->settings                       = $settings;
 		$this->accessSettings                 = new AccessSettings();
 		$this->paymentController              = new PaymentController( $payments, $paymentService );
+		$this->paymentEmailController         = new PaymentEmailController();
 		$this->paymentReportController        = new PaymentReportController( $payments );
 	}
 
@@ -105,17 +114,26 @@ class AdminMenu {
 	 * @param string $hookSuffix Screen the enqueue is running for.
 	 */
 	public function enqueueAssets( string $hookSuffix ): void {
-		if ( '' === $this->availabilityHook || $hookSuffix !== $this->availabilityHook ) {
+		if ( '' !== $this->availabilityHook && $hookSuffix === $this->availabilityHook ) {
+			wp_enqueue_script(
+				'us-scheduler-availability-admin',
+				USC_PLUGIN_URL . 'assets/js/availability-admin.js',
+				[],
+				USC_VERSION,
+				true
+			);
 			return;
 		}
 
-		wp_enqueue_script(
-			'us-scheduler-availability-admin',
-			USC_PLUGIN_URL . 'assets/js/availability-admin.js',
-			[],
-			USC_VERSION,
-			true
-		);
+		if ( '' !== $this->paymentEmailHook && $hookSuffix === $this->paymentEmailHook ) {
+			wp_enqueue_script(
+				'us-scheduler-payment-email-admin',
+				USC_PLUGIN_URL . 'assets/js/payment-email-admin.js',
+				[],
+				USC_VERSION,
+				true
+			);
+		}
 	}
 
 	public function addPages(): void {
@@ -263,6 +281,16 @@ class AdminMenu {
 			30
 		);
 
+		// Studio admin: view, edit and preview the payment-due email template.
+		$this->paymentEmailHook = (string) add_submenu_page(
+			'us-settings',
+			__( 'Payment Due Email', 'unsupervised-schedular' ),
+			__( 'Payment Due Email', 'unsupervised-schedular' ),
+			RoleManager::CAP_MANAGE_BILLING,
+			'us-payment-email',
+			[ $this->paymentEmailController, 'renderPage' ]
+		);
+
 		// Site owner: whether WordPress administrators are studio admins / instructors.
 		// Gated on the core manage_options capability — never the plugin's own grants —
 		// so an administrator can always reach it to re-enable a disabled grant.