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/Plugin.php

@@ -10,6 +10,7 @@ use Unsupervised\Schedular\Auth\RegistrationLoginGate;
 use Unsupervised\Schedular\Auth\RegistrationMailer;
 use Unsupervised\Schedular\Auth\RegistrationPage;
 use Unsupervised\Schedular\Auth\RoleManager;
+use Unsupervised\Schedular\Auth\StudentAdminGuard;
 use Unsupervised\Schedular\Booking\BookingPage;
 use Unsupervised\Schedular\Availability\AvailabilityRepository;
 use Unsupervised\Schedular\Booking\BookingRepository;
@@ -96,6 +97,7 @@ class Plugin {
 		( new UpdateChecker() )->register();
 		( new RoleManager() )->register();
 		( new RegistrationLoginGate() )->register();
+		( new StudentAdminGuard() )->register();
 		( new EmailConfirmationHandler( $settings, $registrationMailer ) )->register();
 		( new AdminMenu( $availability, $bookings, $offerings, $questions, $answers, $policies, $policyVersions, $policyService, $acceptances, $invites, $enrollments, $groupAccess, $settings, $paymentRepo, $paymentService, $resolver, $registrationMailer, $creditRepo ) )->register();
 		( new RestRegistrar( $availability, $bookings, $offerings, $questions, $policies, $policyVersions, $policyService, $registrationGate, $enrollments, $groupAccess, $paymentService ) )->register();