Grant studio-admin capabilities to WordPress administrators

WordPress administrators (manage_options) now implicitly hold every
studio-admin capability via a user_has_cap filter, so the site owner runs
the studio without being assigned the separate us_studio_admin role. The
grant persists nothing and is removed on deactivation. The us_studio_admin
role still exists for non-administrator staff and does NOT confer any core
WordPress admin powers.

Also re-gate the studio-wide "Scheduler" dashboard off manage_options onto
a new view_all_lessons capability (added to the studio-admin cap set), so a
us_studio_admin user can see it too — previously it was administrator-only.

- RoleManager: STUDIO_ADMIN_CAPS constant, CAP_VIEW_ALL_LESSONS,
  grantStudioCapsToAdministrators() user_has_cap filter
- AdminMenu + LessonController: Scheduler gated on view_all_lessons
- Docs: user-roles.md cap matrix + administrator note; lesson-booking.md
- Tests: administrators receive studio caps; non-admins do not

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

src/AdminMenu.php

@@ -32,11 +32,11 @@ class AdminMenu {
 	}
 
 	public function addPages(): void {
-		// Admin-only dashboard: all upcoming lessons.
+		// Studio-wide dashboard: all upcoming lessons across instructors.
 		add_menu_page(
 			__( 'Scheduler', 'unsupervised-schedular' ),
 			__( 'Scheduler', 'unsupervised-schedular' ),
-			'manage_options',
+			RoleManager::CAP_VIEW_ALL_LESSONS,
 			'us-scheduler',
 			[ $this->lessonController, 'renderAdminDashboard' ],
 			'dashicons-calendar-alt',