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]>

docs/features/group-classes.md

@@ -62,11 +62,34 @@ deadline (and capacity) so a **late enrolment** can be added after the class has
 closed. Past the deadline the details page labels these as late enrolments. See
 **Admin Interface** below.
 
+## Withdrawal Flow
+A student may withdraw themselves from a class they are enrolled in through the same
+group-class page: an active enrolment shows a **Withdraw** button.
+`POST /enrollments/{id}/withdraw` marks the enrolment `cancelled` (freeing its
+capacity seat) and voids any still-pending payment. It **never issues an account
+credit** — a timely withdrawal is a clean exit, not a refund (credits are reserved
+for cancelled lessons; see `credits.md`).
+
+Self-withdrawal is bounded by the class's **withdrawal deadline** (the instructor's
+`withdrawal_deadline`; see `offerings.md`). Unlike the enrolment deadline it has no
+implicit default — a class with no deadline set stays open to withdrawal for its
+whole life. Past the deadline `POST /enrollments/{id}/withdraw` rejects the request
+with `403 withdrawal_closed`, and the class card shows "Withdrawal has closed —
+contact the studio to withdraw." in place of the Withdraw button. The endpoint also
+returns `404 not_found` for an unknown enrolment and `403 forbidden` when the
+enrolment is not the caller's own; a withdrawal of an already-cancelled enrolment is
+idempotent.
+
+The deadline only bounds student **self**-withdrawal. A studio admin can withdraw a
+student at any time from the **student detail page** (`Auth\StudentActions::withdrawEnrollment`),
+which is never subject to the deadline.
+
 ## REST API
-| Method   | Endpoint                                     | Permission                       |
-|----------|----------------------------------------------|----------------------------------|
-| `GET`    | `/wp-json/us-scheduler/v1/enrollments`       | Any logged-in user               |
-| `POST`   | `/wp-json/us-scheduler/v1/enrollments`       | `book_lesson`                    |
+| Method   | Endpoint                                        | Permission                       |
+|----------|-------------------------------------------------|----------------------------------|
+| `GET`    | `/wp-json/us-scheduler/v1/enrollments`          | Any logged-in user               |
+| `POST`   | `/wp-json/us-scheduler/v1/enrollments`          | `book_lesson`                    |
+| `POST`   | `/wp-json/us-scheduler/v1/enrollments/{id}/withdraw` | Owner (the enrolled student) |
 
 `POST /enrollments` body: `offering_id`, `answers[]` (`question_id` → value),
 `accepted_policy_version_ids[]`, and payment data (see `payments.md`). The