feat(self-service): trek aanvraag in — withdrawal action (S-11c-2, closes #12) (#93)

## What & why

Final sub-slice of **S-11 · Withdrawal (Flow 3)** — the user-facing "trek aanvraag in" action, which **closes #12**.

- **self-service portal**: the submit confirmation gains a **"Trek aanvraag in"** button. It withdraws the just-submitted registration via `postSelfServiceRegistrationsIdWithdraw(reference)`; success shows an *ingetrokken* confirmation, a failure is surfaced (`role="alert"`) and the action stays available — same confirm-and-surface pattern as submit.
- **acceptance**: `Een registratie intrekken` — owner withdraws → INGETROKKEN + workflow cancelled; a different bsn is reported not-found.
- **e2e**: `withdrawal.spec.ts` — DigiD submit → trek aanvraag in → the portal confirms ingetrokken.
- **docs**: demo-script + frontend-decisions.

Together with S-11a (#88), S-11b (#89), S-11c-1 (#90), this completes the flow: citizen withdraws → domain INGETROKKEN → BPMN message event cancels the process → the case leaves the behandelaar's werkbak.

Closes #12

## Definition of Done

- [x] Linked Gitea issue (#12).
- [x] Failing tests committed before the implementation.
- [x] Implementation makes the tests pass.
- [x] Conventional Commits referencing the issue (`refs #12`).
- [ ] CI green — all Gitea Actions jobs.
- [x] `docker compose up` unaffected.
- [x] Docs updated (demo-script + frontend-decisions).
- [x] ADR — ADR-0014 (from S-11b) covers the cancellation decision; nothing new here.

## Notes for reviewers

- Full local gate run before pushing: `dotnet format --verify-no-changes` clean; `make unit` green (Acceptance **11** incl. the 2 new withdrawal scenarios, Big 95, BFF 30, Acl 27, EventSubscriber 19); self-service lint/test/build green (9 tests, incl. the 2 new withdraw tests).
- `withdrawal.spec.ts` waits on the *ingetrokken* confirmation (which only renders after the withdraw POST returns), so it can't cancel the request early (the 499 lesson from #87). Live-validated by verify-stack.

Reviewed-on: #93

apps/self-service/src/app/registration/registration-page.spec.ts

@@ -17,12 +17,22 @@ class FakeAuth extends AuthService {
   }
 }
 
-function providers(post = vi.fn().mockReturnValue(of({ registrationId: 'reg-9', status: 'Ingediend' }))) {
+function providers(
+  post = vi.fn().mockReturnValue(of({ registrationId: 'reg-9', status: 'Ingediend' })),
+  withdraw = vi.fn().mockReturnValue(of(undefined)),
+) {
   return {
     post,
+    withdraw,
     providers: [
       { provide: AuthService, useClass: FakeAuth },
-      { provide: BffApiV1Service, useValue: { postSelfServiceRegistrations: post } },
+      {
+        provide: BffApiV1Service,
+        useValue: {
+          postSelfServiceRegistrations: post,
+          postSelfServiceRegistrationsIdWithdraw: withdraw,
+        },
+      },
     ],
   };
 }
@@ -56,6 +66,36 @@ describe('RegistrationPage', () => {
     expect(screen.getByRole('button', { name: /indienen/i })).toBeTruthy();
   });
 
+  it('offers to withdraw after submitting, and withdrawing confirms', async () => {
+    const { withdraw, providers: p } = providers();
+    await render(RegistrationPage, { providers: p });
+
+    fireEvent.click(screen.getByRole('button', { name: /indienen/i }));
+    await screen.findByText(/ontvangen/i);
+
+    fireEvent.click(await screen.findByRole('button', { name: /trek aanvraag in/i }));
+
+    // The withdrawal is keyed by the reference the submit returned, and the page confirms it.
+    expect(withdraw).toHaveBeenCalledWith('reg-9');
+    expect(await screen.findByText(/ingetrokken/i)).toBeTruthy();
+  });
+
+  it('surfaces a withdraw failure and keeps the action available', async () => {
+    const { providers: p } = providers(
+      vi.fn().mockReturnValue(of({ registrationId: 'reg-9', status: 'Ingediend' })),
+      vi.fn().mockReturnValue(throwError(() => new Error('withdraw rejected'))),
+    );
+    await render(RegistrationPage, { providers: p });
+
+    fireEvent.click(screen.getByRole('button', { name: /indienen/i }));
+    await screen.findByText(/ontvangen/i);
+    fireEvent.click(await screen.findByRole('button', { name: /trek aanvraag in/i }));
+
+    expect(await screen.findByRole('alert')).toBeTruthy();
+    expect(screen.queryByText(/is ingetrokken/i)).toBeNull();
+    expect(screen.getByRole('button', { name: /trek aanvraag in/i })).toBeTruthy();
+  });
+
   it('has no WCAG 2.1 AA violations on the submit page', async () => {
     // The portal is Dutch; the real index.html sets lang. Set it here so the document-level
     // html-has-lang rule reflects the app, not the bare jsdom document.