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

docs/frontend-decisions.md

@@ -151,3 +151,19 @@ frontend work is the medewerker realm auth and the werkbak/decide page. Wiring r
   to assert the medewerker token attaches to `/behandel/*` (and not to the anonymous openbaar call).
   The full DigiD-submit → behandel-decide → public INGESCHREVEN round-trip is the Playwright happy
   path.
+
+## Self-service withdrawal: "trek aanvraag in" (S-11c, #12)
+
+The submit confirmation grows a **"Trek aanvraag in"** action so a zorgprofessional can withdraw the
+registration they just submitted (`apps/self-service`, on the existing `RegistrationPage`).
+
+- **Keyed by the reference, owner-scoped at the BFF.** The button calls the generated
+  `postSelfServiceRegistrationsIdWithdraw(reference)` with the reference the submit returned. The
+  DigiD token (attached by the interceptor) carries the bsn the BFF forwards; the domain only lets
+  the owner withdraw (a mismatch is 404). No extra identity is entered in the UI.
+- **Same confirm-and-surface pattern as submit.** A secondary-action button; on success the page
+  switches to an ingetrokken confirmation; a failure is surfaced (`role="alert"`) and the action
+  stays available to retry — mirroring how submit handles its failure rather than swallowing it.
+- **Testing.** Component tests (`@testing-library/angular`, mocked BFF) cover the button appearing
+  after submit, the reference being passed, the ingetrokken confirmation, and the failure path; the
+  browser round-trip is `tests/e2e/withdrawal.spec.ts`.