docs(withdrawal): demo-script + frontend decision for 'trek aanvraag in' (refs #12)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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`.