Let the studio register the students who cannot register themselves

The Book a lesson for a student panel built its picker from the us_student
role but vetted the submission with the book_lesson capability. ChildLoginGate
and RegistrationLoginGate withhold that capability from accounts that keep the
role, so the panel offered every guardian-managed child and every unapproved
signup and then refused them — with a message claiming no student had been
chosen, and a form cleared of all five fields.

Withholding book_lesson stops those accounts registering in their own name. It
was never meant to stop the studio acting for them, which is what the panel is
for, and for a child is the only route to a lesson besides their guardian.

Guard the student role instead, via a new RoleManager::isStudent() shared with
every picker and guard on the staff side so the two cannot drift apart again.
Group enrolment gets the same predicate: addDirect() and grantAccess() vetted
their posted ids not at all, and would enrol an instructor, an administrator,
or an account deleted since the page was drawn — raising a real payment against
them for a priced class.

Keep a refused booking's fields as submitted, reading the form through one
LessonController::submittedBooking() so what gets booked and what is shown
again cannot disagree about a field name. A booking that succeeds still leaves
an empty form, so the next one does not inherit it.

Closes #185

Co-Authored-By: Claude Opus 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01XunBYk2sFEc1oL14sUiuBU

src/Auth/RoleManager.php

@@ -60,6 +60,27 @@ class RoleManager {
 		self::CAP_EXPORT_PAYMENTS,
 	];
 
+	/**
+	 * Whether a user account is a student the studio may act for.
+	 *
+	 * Deliberately the role and not the `book_lesson` capability: that capability
+	 * is withheld from a guardian's child ({@see \Unsupervised\Schedular\Guardian\ChildLoginGate})
+	 * and from a self-signup still awaiting approval
+	 * ({@see \Unsupervised\Schedular\Auth\RegistrationLoginGate}), so that neither
+	 * can book or enrol *in their own name*. Staff booking or enrolling on their
+	 * behalf is the case those restrictions exist to leave open — and for a child,
+	 * whose account is never signed in to, it is the only route there is.
+	 *
+	 * Use this for every "may the studio register this person?" check, so the
+	 * pickers staff choose from and the guards that vet their choice cannot drift
+	 * into offering someone who is then refused.
+	 */
+	public static function isStudent( int $userId ): bool {
+		$user = $userId > 0 ? get_userdata( $userId ) : false;
+
+		return $user instanceof \WP_User && in_array( self::STUDENT, (array) $user->roles, true );
+	}
+
 	public function __construct( private AccessSettings $access = new AccessSettings() ) {}
 
 	public function register(): void {