Fix field-length saves, student wp-admin access, and empty instructor picker

Three bug fixes for the 1.2.1 section:

- Fixed-size fields (question labels, offering titles/notes/e-transfer
  email, policy titles/slugs) no longer silently fail to save when the
  value exceeds its column length. The REST endpoints reject over-long
  values with a 400, the admin controllers refuse to insert them, and the
  form inputs carry a maxlength so the browser blocks over-long entry.
  Limits are MAX_* constants on the value objects, kept in lockstep with
  the schema columns.

- Students are kept out of wp-admin entirely. New StudentAdminGuard
  redirects front-end-only users (no back-office capability) away from the
  dashboard and hides the admin bar for them, while administrators, studio
  admins, and instructors keep full access.

- The Add/Edit Offering instructor picker now includes WordPress
  administrators when they act as instructors (the default single-account
  setup), so a solo studio owner is selectable instead of the dropdown
  being empty.

composer test (618), composer lint, composer cs all pass.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/Offering/OfferingController.php

@@ -3,6 +3,7 @@ declare(strict_types=1);
 
 namespace Unsupervised\Schedular\Offering;
 
+use Unsupervised\Schedular\Auth\AccessSettings;
 use Unsupervised\Schedular\Auth\RoleManager;
 use Unsupervised\Schedular\Val;
 
@@ -11,6 +12,7 @@ class OfferingController {
 	public function __construct(
 		private OfferingRepository $repository,
 		private ClassSlotReconciler $reconciler,
+		private AccessSettings $access = new AccessSettings(),
 	) {}
 
 	public function renderPage(): void {
@@ -137,17 +139,28 @@ class OfferingController {
 	}
 
 	/**
-	 * Registered instructors offered in the assignment select, by display name.
+	 * Instructors offered in the assignment select, by display name.
+	 *
+	 * Includes everyone holding the `us_instructor` role plus, when the site owner
+	 * has left administrators acting as instructors (the default single-account
+	 * setup), WordPress administrators — who teach through the dynamic capability
+	 * grant rather than the role. Without them a solo studio owner running the
+	 * business from an admin account would find no one to assign a class to.
 	 *
 	 * @return list<array{id: int, name: string}>
 	 */
 	private function instructorOptions(): array {
+		$roles = [ RoleManager::INSTRUCTOR ];
+		if ( $this->access->adminsAreInstructors() ) {
+			$roles[] = 'administrator';
+		}
+
 		$users = array_filter(
 			get_users(
 				[
-					'role'    => RoleManager::INSTRUCTOR,
-					'orderby' => 'display_name',
-					'order'   => 'ASC',
+					'role__in' => $roles,
+					'orderby'  => 'display_name',
+					'order'    => 'ASC',
 				]
 			),
 			static fn( mixed $u ): bool => $u instanceof \WP_User
@@ -184,6 +197,17 @@ class OfferingController {
 			return null;
 		}
 
+		$scheduleNote   = $this->nullableText( sanitize_text_field( Val::string( wp_unslash( $_POST['schedule_note'] ?? '' ) ) ) );
+		$etransferEmail = $this->nullableText( sanitize_email( Val::string( wp_unslash( $_POST['etransfer_email'] ?? '' ) ) ) );
+
+		// Reject over-long fixed-size fields rather than let the DB silently drop them.
+		if ( mb_strlen( $title ) > Offering::MAX_TITLE_LENGTH
+			|| ( null !== $scheduleNote && mb_strlen( $scheduleNote ) > Offering::MAX_SCHEDULE_NOTE_LENGTH )
+			|| ( null !== $etransferEmail && mb_strlen( $etransferEmail ) > Offering::MAX_ETRANSFER_EMAIL_LENGTH )
+		) {
+			return null;
+		}
+
 		$billingMode = sanitize_key( Val::string( wp_unslash( $_POST['billing_mode'] ?? Offering::BILLING_ONE_TIME ) ) );
 		if ( ! in_array( $billingMode, Offering::VALID_BILLING_MODES, true ) ) {
 			$billingMode = Offering::BILLING_ONE_TIME;
@@ -234,8 +258,8 @@ class OfferingController {
 			classTime:       $classTime,
 			enrollmentDeadline: $enrollmentDeadline,
 			withdrawalDeadline: $withdrawalDeadline,
-			scheduleNote:    $this->nullableText( sanitize_text_field( Val::string( wp_unslash( $_POST['schedule_note'] ?? '' ) ) ) ),
-			etransferEmail:  $this->nullableText( sanitize_email( Val::string( wp_unslash( $_POST['etransfer_email'] ?? '' ) ) ) ),
+			scheduleNote:    $scheduleNote,
+			etransferEmail:  $etransferEmail,
 			cancellationCutoffHours: $cutoffHours,
 			accessMode:      isset( $_POST['invite_only'] ) ? Offering::ACCESS_INVITE_ONLY : Offering::ACCESS_PUBLIC,
 			isActive:        isset( $_POST['is_active'] ),