feat(fp): brief v3 — besluit-driven guided drafting

Compose the herregistratie letter from the besluit instead of a library hunt:
the behandelaar picks positief/negatief (+ reden-checkboxes for a negatief) and
the kern's standaardteksten follow the selection live.

Front-end (this increment):
- Kern is recomposed reactively from the besluit selection (new BesluitSelected
  machine msg + composeKern); the "Genereer conceptbrief" button is gone. The
  drafter's free text is preserved across a selection change.
- The editor shows only the editable sections; the locked aanhef/slot render in
  the preview, not the authoring surface. Slot is a case-type template section
  (per templateId), documented as such.
- The panel re-seeds from the letter via inferSelection() — the besluit + redenen
  are read back off the kern's passage blocks, so the selection survives reload
  with no new wire fields (derive, don't store).
- letter-section drops the now-redundant per-section passage picker (besluit owns
  standaardteksten); keeps free-text + block edit/move/remove.

Fix: app-checkbox now falls back to a unique per-instance id when checkboxId is
omitted. The CIBG styled checkbox routes clicks through the label, so the shared
id="undefined" made every reason label toggle the first input — the second
checkbox could never be checked. Verified live (Playwright): each reason toggles
independently.

Backend/seam (brief v3 WIP): besluit/reason passage tags on the wire + seed,
carried through the adapter parse boundary.

Specs updated (besluit, brief.machine) and the affected stories re-pointed at the
new API. FE lint + build + 253 vitest specs green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/BigRegister.Api/Program.cs

@@ -450,7 +450,10 @@ BriefViewDto ToView(HttpContext ctx, BriefEntity e) => new(
     Authz.Decisions(Authz.ResolvePrincipal(ctx), e.Status.Tag, e.DrafterId),
     // Sent letters render with the version pinned at send; everything else follows
     // the sub-org's current published template (WP-23 immutability invariant).
-    OrgTemplateStore.TemplateForBrief(e.SubOrgId, e.Status.Tag == "sent" ? e.SentOrgTemplateVersion : null));
+    OrgTemplateStore.TemplateForBrief(e.SubOrgId, e.Status.Tag == "sent" ? e.SentOrgTemplateVersion : null),
+    // The case this letter is about — joined from the seeded zorgverlener so the
+    // behandel scherm can show whom/what it concerns without brief/ importing registratie.
+    new CaseContextDto(SeedData.Registration.Naam, SeedData.Registration.BigNummer, e.Beroep, BriefSeed.AanvraagReferentie));
 
 // Emit (decision flags, via ToView) and enforce (Forbidden/Conflict below) both run
 // through Authz — see BriefStore.Review and Authz.CanActOn — so they cannot drift.