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/Auth/StudentAdminGuard.php

@@ -0,0 +1,103 @@
+<?php
+declare(strict_types=1);
+
+namespace Unsupervised\Schedular\Auth;
+
+/**
+ * Keeps front-end-only users (students) out of wp-admin entirely.
+ *
+ * Students authenticate through the front-end login shortcode and do all of
+ * their work — booking, viewing lessons, paying — on the site's public pages.
+ * They have no reason to see the WordPress dashboard, profile screen, or admin
+ * bar, so this guard redirects them to the front end if they reach wp-admin and
+ * hides the admin bar for them everywhere.
+ *
+ * Access is decided by capability, not role: anyone holding a back-office
+ * capability (a WordPress administrator, studio admin, or instructor) keeps full
+ * wp-admin access, while a user with none of them is treated as front-end only.
+ */
+class StudentAdminGuard {
+
+	/**
+	 * Capabilities that grant a genuine reason to be in wp-admin. A user holding
+	 * none of these is front-end only and is kept out of the dashboard.
+	 *
+	 * @var list<string>
+	 */
+	private const BACK_OFFICE_CAPS = [
+		'manage_options',
+		RoleManager::CAP_MANAGE_INSTRUCTORS,
+		RoleManager::CAP_MANAGE_STUDENTS,
+		RoleManager::CAP_MANAGE_OFFERINGS,
+		RoleManager::CAP_MANAGE_QUESTIONS,
+		RoleManager::CAP_MANAGE_POLICIES,
+		RoleManager::CAP_MANAGE_BILLING,
+		RoleManager::CAP_MANAGE_AVAILABILITY,
+		RoleManager::CAP_VIEW_ALL_LESSONS,
+		RoleManager::CAP_VIEW_ALL_PAYMENTS,
+		RoleManager::CAP_VIEW_OWN_PAYMENTS,
+		RoleManager::CAP_EXPORT_PAYMENTS,
+	];
+
+	public function register(): void {
+		add_action( 'admin_init', [ $this, 'redirectFromDashboard' ] );
+		add_filter( 'show_admin_bar', [ $this, 'hideAdminBar' ] );
+	}
+
+	/**
+	 * Redirect a front-end-only user away from any wp-admin page to the site
+	 * home, so the dashboard and profile screens are never reachable.
+	 */
+	public function redirectFromDashboard(): void {
+		if ( ! $this->shouldBlockAdminAccess() ) {
+			return;
+		}
+
+		wp_safe_redirect( home_url( '/' ) );
+		exit;
+	}
+
+	/**
+	 * Whether the current request into wp-admin should be bounced to the front
+	 * end. AJAX requests are always allowed through so front-end features that
+	 * call admin-ajax keep working.
+	 */
+	public function shouldBlockAdminAccess(): bool {
+		if ( wp_doing_ajax() ) {
+			return false;
+		}
+
+		if ( ! is_user_logged_in() ) {
+			return false;
+		}
+
+		return ! $this->hasBackOfficeAccess();
+	}
+
+	/**
+	 * Hide the admin bar for front-end-only users; leave it untouched for anyone
+	 * with back-office access.
+	 *
+	 * @param bool $show Whether WordPress would otherwise show the admin bar.
+	 */
+	public function hideAdminBar( bool $show ): bool {
+		if ( is_user_logged_in() && ! $this->hasBackOfficeAccess() ) {
+			return false;
+		}
+
+		return $show;
+	}
+
+	/**
+	 * Whether the current user holds any capability that warrants wp-admin access.
+	 */
+	private function hasBackOfficeAccess(): bool {
+		foreach ( self::BACK_OFFICE_CAPS as $cap ) {
+			if ( current_user_can( $cap ) ) {
+				return true;
+			}
+		}
+
+		return false;
+	}
+}