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/Policy/PolicyEndpoint.php

@@ -118,12 +118,30 @@ class PolicyEndpoint {
 		if ( '' === $title ) {
 			return $this->invalid( __( 'A policy title is required.', 'unsupervised-schedular' ) );
 		}
+		if ( mb_strlen( $title ) > Policy::MAX_TITLE_LENGTH ) {
+			return $this->invalid(
+				sprintf(
+					/* translators: %d: maximum character count. */
+					__( 'The policy title must be %d characters or fewer.', 'unsupervised-schedular' ),
+					Policy::MAX_TITLE_LENGTH
+				)
+			);
+		}
 
 		$slugParam = sanitize_text_field( Val::string( $request->get_param( 'slug' ) ) );
 		$slug      = sanitize_title( '' !== $slugParam ? $slugParam : $title );
 		if ( '' === $slug ) {
 			return $this->invalid( __( 'A valid policy slug is required.', 'unsupervised-schedular' ) );
 		}
+		if ( mb_strlen( $slug ) > Policy::MAX_SLUG_LENGTH ) {
+			return $this->invalid(
+				sprintf(
+					/* translators: %d: maximum character count. */
+					__( 'The policy slug must be %d characters or fewer.', 'unsupervised-schedular' ),
+					Policy::MAX_SLUG_LENGTH
+				)
+			);
+		}
 
 		if ( null !== $this->policies->findBySlug( $slug ) ) {
 			return new \WP_Error( 'duplicate_slug', __( 'A policy with that slug already exists.', 'unsupervised-schedular' ), [ 'status' => 409 ] );