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]>

tests/Unit/Offering/OfferingRepositoryTest.php

@@ -192,6 +192,29 @@ class OfferingRepositoryTest extends TestCase
         self::assertSame(1, $this->repo->insert($offering));
     }
 
+    public function testInsertPersistsWithdrawalDeadline(): void
+    {
+        Functions\expect('current_time')->with('mysql')->andReturn('2026-04-01 12:00:00');
+
+        $this->db->shouldReceive('insert')
+            ->once()
+            ->with(
+                'wp_us_offerings',
+                Mockery::on(static fn (array $data): bool => $data['withdrawal_deadline'] === '2026-08-31'),
+                Mockery::type('array')
+            );
+        $this->db->insert_id = 1;
+
+        $offering = new Offering(
+            instructorId:       5,
+            kind:               Offering::KIND_GROUP_CLASS,
+            title:              'Choir',
+            withdrawalDeadline: '2026-08-31',
+        );
+
+        self::assertSame(1, $this->repo->insert($offering));
+    }
+
     public function testDeleteCallsWpdbDelete(): void
     {
         $this->db->shouldReceive('delete')