Add group-class withdrawal deadline and kind-aware offering form

Group classes now carry an optional per-class withdrawal deadline. Up to
that day a student may withdraw themselves from the class; the withdrawal
frees the seat and voids any pending payment but never issues an account
credit. After the deadline self-withdrawal closes and a studio admin must
withdraw the student by hand (the admin path is never subject to the
deadline). A blank deadline keeps self-withdrawal open indefinitely.

Also make the Add/Edit Offering form show only the fields relevant to the
selected kind: group settings for group classes, weekly reservation for
private lessons. Progressive enhancement — without JS every field renders.

- New nullable us_offerings.withdrawal_deadline column; Offering model gains
  $withdrawalDeadline + isWithdrawalOpen().
- New student endpoint POST /enrollments/{id}/withdraw, gated by the deadline
  (403 withdrawal_closed), ownership-checked, idempotent.
- Front-end group-class page shows a Withdraw button while open.
- No USC_VERSION bump: 1.2.0 is unreleased and accumulates schema changes
  under its section, matching the scheduled-billing and credit features.

Tests: composer test (596), composer lint, composer cs all pass.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/Offering/OfferingController.php

@@ -213,6 +213,11 @@ class OfferingController {
 		// day (term_start), applied by Offering::effectiveEnrollmentDeadline().
 		$enrollmentDeadline = Offering::normalizeDate( sanitize_text_field( Val::string( wp_unslash( $_POST['enrollment_deadline'] ?? '' ) ) ) );
 
+		// A blank (or invalid) withdrawal deadline leaves the column NULL, which
+		// keeps self-withdrawal open for the class's whole life
+		// (Offering::isWithdrawalOpen()). A set date closes it after that day.
+		$withdrawalDeadline = Offering::normalizeDate( sanitize_text_field( Val::string( wp_unslash( $_POST['withdrawal_deadline'] ?? '' ) ) ) );
+
 		return new Offering(
 			instructorId:    $this->resolveInstructorId( $instructorId, $manageAll, $existing ),
 			kind:            $kind,
@@ -228,6 +233,7 @@ class OfferingController {
 			termEnd:         $termEnd,
 			classTime:       $classTime,
 			enrollmentDeadline: $enrollmentDeadline,
+			withdrawalDeadline: $withdrawalDeadline,
 			scheduleNote:    $this->nullableText( sanitize_text_field( Val::string( wp_unslash( $_POST['schedule_note'] ?? '' ) ) ) ),
 			etransferEmail:  $this->nullableText( sanitize_email( Val::string( wp_unslash( $_POST['etransfer_email'] ?? '' ) ) ) ),
 			cancellationCutoffHours: $cutoffHours,