Stop the availability form failing in silence

Adding availability for 5:30-6:00 PM with the lesson length left on its
60-minute default saved nothing and said nothing. A window is stored as
consecutive lesson-length slots, so one that fits no lesson splits into
none: splitByDuration() returned [], createFromWindow() inserted
nothing, and addSlot() discarded the result and re-rendered the page
unchanged.

The REST endpoint already rejected that window with a 400. The admin
form checked the same rules separately, and its copy was both laxer and
mute — an unreadable date, an end before the start, and a two-day window
were bare `return`s, and it never checked offering ownership at all, so
a crafted POST could tie a slot to another instructor's offering and
inherit their price and payment routing.

Both callers now go through WindowValidator, which returns the window or
a WP_Error explaining the refusal. The endpoint returns that error as
is; the page renders its message as a notice. handleFormAction returns
a [notice, error] pair so deletes report themselves too, and a
successful add says how many slots it created.

Two failures could also go unnoticed underneath: wpdb::insert's result
was ignored, and insert_id still holds the previous statement's id after
a failed write, so a failure looked like a success — and could become
the recurrence group of a weekly series, orphaning every later
occurrence. weeks was unbounded server-side despite the form's max=52.

availability-admin.js narrows the lesson-length choices to those that
fit the window and blocks submission when none do, which is what makes
the original mistake hard to repeat. It is a convenience: the server
validates regardless.

Closes #130

src/AdminMenu.php

@@ -5,6 +5,7 @@ namespace Unsupervised\Schedular;
 
 use Unsupervised\Schedular\Availability\AvailabilityController;
 use Unsupervised\Schedular\Availability\AvailabilityRepository;
+use Unsupervised\Schedular\Availability\WindowValidator;
 use Unsupervised\Schedular\Auth\AccessSettings;
 use Unsupervised\Schedular\Auth\InstructorController;
 use Unsupervised\Schedular\Auth\InviteRepository;
@@ -42,6 +43,12 @@ use Unsupervised\Schedular\Registration\QuestionRepository;
 
 class AdminMenu {
 
+	/**
+	 * Hook suffix of the availability screen, captured when the page is added so
+	 * its script loads on that screen only.
+	 */
+	private string $availabilityHook = '';
+
 	private AvailabilityController $availabilityController;
 	private LessonController $lessonController;
 	private OfferingController $offeringController;
@@ -58,7 +65,7 @@ class AdminMenu {
 	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 ) {
-		$this->availabilityController         = new AvailabilityController( $availability, $offerings );
+		$this->availabilityController         = new AvailabilityController( $availability, $offerings, new WindowValidator( $offerings ) );
 		$this->lessonController               = new LessonController( $bookings, $payments, $availability, $offerings, new LessonDetail( $answers, $questions, $acceptances, $policies, $policyVersions ) );
 		$this->offeringController             = new OfferingController( $offerings, new ClassSlotReconciler( $availability ) );
 		$this->questionController             = new QuestionController( $questions, $offerings );
@@ -76,9 +83,29 @@ class AdminMenu {
 
 	public function register(): void {
 		add_action( 'admin_menu', [ $this, 'addPages' ] );
+		add_action( 'admin_enqueue_scripts', [ $this, 'enqueueAssets' ] );
 		add_action( 'admin_post_' . PaymentReportController::EXPORT_ACTION, [ $this->paymentReportController, 'export' ] );
 	}
 
+	/**
+	 * Load a screen's script on that screen only.
+	 *
+	 * @param string $hookSuffix Screen the enqueue is running for.
+	 */
+	public function enqueueAssets( string $hookSuffix ): void {
+		if ( '' === $this->availabilityHook || $hookSuffix !== $this->availabilityHook ) {
+			return;
+		}
+
+		wp_enqueue_script(
+			'us-scheduler-availability-admin',
+			USC_PLUGIN_URL . 'assets/js/availability-admin.js',
+			[],
+			USC_VERSION,
+			true
+		);
+	}
+
 	public function addPages(): void {
 		$this->addStudioSeparators();
 
@@ -94,7 +121,7 @@ class AdminMenu {
 		);
 
 		// Instructor: manage their own availability.
-		add_menu_page(
+		$this->availabilityHook = (string) add_menu_page(
 			__( 'My Availability', 'unsupervised-schedular' ),
 			__( 'My Availability', 'unsupervised-schedular' ),
 			RoleManager::CAP_MANAGE_AVAILABILITY,