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/Registration/Question.php

@@ -12,6 +12,9 @@ class Question {
 	public const FIELD_SELECT   = 'select';
 	public const FIELD_CHECKBOX = 'checkbox';
 
+	/** Maximum length of a question label, matching the `label` VARCHAR(255) column. */
+	public const MAX_LABEL_LENGTH = 255;
+
 	/** Question is scoped to a single offering, asked at booking/enrolment time. */
 	public const SCOPE_OFFERING = 'offering';