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.html

@@ -3,9 +3,29 @@
     <utrecht-heading-1>Zelfservice — BIG-registratie</utrecht-heading-1>
 
     @if (submitted()) {
-      <p utrecht-paragraph role="status">
-        Uw registratie is ontvangen. Referentie: {{ reference() }}.
-      </p>
+      @if (withdrawn()) {
+        <p utrecht-paragraph role="status">
+          Uw registratie met referentie {{ reference() }} is ingetrokken.
+        </p>
+      } @else {
+        <p utrecht-paragraph role="status">
+          Uw registratie is ontvangen. Referentie: {{ reference() }}.
+        </p>
+        @if (withdrawFailed()) {
+          <p utrecht-paragraph role="alert">
+            Het intrekken van uw registratie is niet gelukt. Probeer het opnieuw.
+          </p>
+        }
+        <button
+          utrecht-button
+          appearance="secondary-action-button"
+          type="button"
+          [disabled]="withdrawing()"
+          (click)="withdraw()"
+        >
+          Trek aanvraag in
+        </button>
+      }
     } @else {
       <p utrecht-paragraph>U bent ingelogd met BSN {{ bsn() }}.</p>
       @if (failed()) {