Make WP admins instructors too, and add an Access toggle page

A WordPress administrator previously inherited the studio-admin
capabilities but not `manage_availability`, so the studio owner running
as an admin had no way to reach "My Availability" or act as the
instructor — breaking single-instructor businesses.

Grant the instructor capabilities to administrators as well (via the
existing `user_has_cap` filter), and make both grants — studio-admin and
instructor — independently toggleable from a new Access admin page.

- RoleManager: extract `INSTRUCTOR_CAPS`; apply studio and instructor
  cap sets to administrators, each gated on a stored toggle (default on).
- AccessSettings + templates/admin/access.php: two options
  (`us_admin_grant_studio` / `us_admin_grant_instructor`), gated on the
  core `manage_options` capability so disabling a grant can never lock an
  administrator out of re-enabling it.
- AdminMenu: register the Access page after Studio Settings; keep the
  studio sidebar separator visible for any administrator.
- Tests for the toggles and the new settings reader; docs updated.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/AdminMenu.php

@@ -5,6 +5,7 @@ namespace Unsupervised\Schedular;
 
 use Unsupervised\Schedular\Availability\AvailabilityController;
 use Unsupervised\Schedular\Availability\AvailabilityRepository;
+use Unsupervised\Schedular\Auth\AccessSettings;
 use Unsupervised\Schedular\Auth\InviteRepository;
 use Unsupervised\Schedular\Auth\RegistrationController;
 use Unsupervised\Schedular\Auth\RoleManager;
@@ -39,6 +40,7 @@ class AdminMenu {
 	private GroupClassController $groupClassController;
 	private StudentController $studentController;
 	private StudioSettings $settings;
+	private AccessSettings $accessSettings;
 	private PaymentController $paymentController;
 	private PaymentReportController $paymentReportController;
 
@@ -52,6 +54,7 @@ class AdminMenu {
 		$this->groupClassController    = new GroupClassController( $enrollments, $offerings );
 		$this->studentController       = new StudentController( $bookings, $availability, $offerings, $enrollments, $resolver );
 		$this->settings                = $settings;
+		$this->accessSettings          = new AccessSettings();
 		$this->paymentController       = new PaymentController( $payments, $paymentService );
 		$this->paymentReportController = new PaymentReportController( $payments );
 	}
@@ -185,6 +188,19 @@ class AdminMenu {
 			30
 		);
 
+		// Site owner: whether WordPress administrators are studio admins / instructors.
+		// Gated on the core manage_options capability — never the plugin's own grants —
+		// so an administrator can always reach it to re-enable a disabled grant.
+		add_menu_page(
+			__( 'Access', 'unsupervised-schedular' ),
+			__( 'Access', 'unsupervised-schedular' ),
+			'manage_options',
+			'us-access',
+			[ $this->accessSettings, 'renderPage' ],
+			'dashicons-admin-network',
+			30.5
+		);
+
 		// Instructor: view their upcoming lessons.
 		add_menu_page(
 			__( 'My Lessons', 'unsupervised-schedular' ),
@@ -234,6 +250,12 @@ class AdminMenu {
 	}
 
 	private function userSeesStudioMenu(): bool {
+		// Administrators always see the Access page, so the leading separator
+		// should show for them even if both capability grants are disabled.
+		if ( current_user_can( 'manage_options' ) ) {
+			return true;
+		}
+
 		$caps = [
 			RoleManager::CAP_VIEW_ALL_LESSONS,
 			RoleManager::CAP_VIEW_LESSONS,