Ask who the signup is for as a three-way choice

Replaces the single "I'm registering as a parent or guardian" tick with
"Just myself" / "On behalf of one or more students" / "Both".

Radios, not checkboxes as the feedback put it: the three answers are
mutually exclusive, and "both" only means anything as a third choice
alongside the other two.

The tick could only ever say whether there were children to add. It could
not say whether the account holder was a student, so bookableStudents()
always offered them their own name and any guardian could book themselves a
lesson nobody meant to sell. "On behalf of" now records us_guardian_only and
leaves them out of the picker.

That flag is stored as the negative on purpose. Every account predating this
choice is a bookable student, and absence has to keep meaning exactly that,
or the picker would quietly stop offering people themselves on upgrade.
setGuardianOnly() clears the key rather than writing 0, so "not set" stays
the single spelling of "yes, a student". A guardian-only account with nobody
linked to it is still offered itself — an empty picker is no way to book at
all, and they can put the account right from the profile page.

An unrecognised or absent value reads as "just myself": the choice that
collects the least and grants the least. A missing radio must never be taken
as "register these children".

Bumps to 1.4.0.

The account holder's own questions stay out of play whenever students are
being added, "both" included — asking them there is #146.

Verified the form in a headless browser across all three choices: which
blocks show, which fields carry `required`, whether the account holder's
question panel is disabled, which submit is offered, and that switching back
to "just myself" leaves no hidden required field blocking submit.

Closes #145

Co-Authored-By: Claude Opus 5 <[email protected]>

src/Auth/RegistrationPage.php

@@ -19,6 +19,15 @@ use Unsupervised\Schedular\Val;
 
 class RegistrationPage {
 
+	/** "Who are you registering?": the account holder, and nobody else. */
+	public const FOR_SELF = 'self';
+
+	/** Only other people — the account holder is not a student. */
+	public const FOR_STUDENTS = 'students';
+
+	/** The account holder *and* other people. */
+	public const FOR_BOTH = 'both';
+
 	/** Success signal: an invited student was created and logged in. */
 	private const RESULT_INVITE = 'invite';
 
@@ -306,14 +315,17 @@ class RegistrationPage {
 		// Registering as a parent/guardian turns the account-signup questions from
 		// "about you" into "about each child" — they describe the student
 		// (instrument, level, school), not the person holding the account.
-		$isGuardian = $this->submittedIsGuardian();
+		$registeringFor = $this->submittedRegisteringFor();
+
+		// "Students" and "both" both collect student blocks; only "self" does not.
+		$isGuardian = self::FOR_SELF !== $registeringFor;
 		$children   = $isGuardian ? $this->submittedChildren() : [];
 		$answers    = $isGuardian ? [] : $this->submittedAnswers();
 
 		// Everything is validated before a single user is created, so a bad child
 		// block never leaves a half-registered family behind.
 		if ( $isGuardian && [] === $children ) {
-			return esc_html__( 'Please add at least one student, or uncheck the parent/guardian option.', 'unsupervised-schedular' );
+			return esc_html__( 'Please add at least one student, or choose "Just myself" instead.', 'unsupervised-schedular' );
 		}
 
 		// Name and birth year are required per student, and are checked here for
@@ -360,6 +372,10 @@ class RegistrationPage {
 
 		$this->recordAcceptances( $policyForms, (int) $userId, (int) $userId );
 
+		// Only "students" means the account holder is not a student themselves;
+		// "both" registers them alongside the people they book for.
+		$this->guardians->setGuardianOnly( (int) $userId, self::FOR_STUDENTS === $registeringFor );
+
 		if ( $isGuardian ) {
 			$failure = $this->createChildren( $children, $accountQuestions, $policyForms, (int) $userId );
 			if ( '' !== $failure ) {
@@ -508,12 +524,19 @@ class RegistrationPage {
 	}
 
 	/**
-	 * Whether the "I'm registering as a parent or guardian" box was ticked.
+	 * Who this signup is for: {@see FOR_SELF}, {@see FOR_STUDENTS} or
+	 * {@see FOR_BOTH}.
+	 *
+	 * Anything unrecognised — including a form posted without the field at all —
+	 * falls back to "just myself", the choice that collects the least and grants
+	 * the least. A missing radio must not be read as "register these children".
 	 */
-	private function submittedIsGuardian(): bool {
+	private function submittedRegisteringFor(): string {
 		// The submit nonce is verified by the caller before this runs.
 		// phpcs:ignore WordPress.Security.NonceVerification.Missing
-		return '1' === sanitize_text_field( Val::string( wp_unslash( $_POST['us_is_guardian'] ?? '' ) ) );
+		$value = sanitize_key( Val::string( wp_unslash( $_POST['us_registering_for'] ?? '' ) ) );
+
+		return in_array( $value, [ self::FOR_STUDENTS, self::FOR_BOTH ], true ) ? $value : self::FOR_SELF;
 	}
 
 	/**