feat(fp): WP-25 — server-rendered letter HTML preview

Adds LetterHtml.Render, a pure composer mirroring the FE letter canvas'
class vocabulary, behind two ExcludeFromDescription()'d endpoints
(GET /brief/preview, GET /admin/org-template/{subOrgId}/preview).
Auto-resolvable placeholders pull from seed/case data; unresolved
manual ones render as "[NOG IN TE VULLEN: label]". A sent brief
archives its composed HTML (BriefEntity.ArchivedHtml) so a later
org-template republish never changes it. FE gets a hand-written fetch
adapter (text/html, not JSON) and a "Voorbeeld" button that opens the
preview in a new tab.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/app/brief/application/brief.store.ts

@@ -12,6 +12,7 @@ import {
 import { BriefMsg, BriefState, initial, reduce } from '@brief/domain/brief.machine';
 import { OrgTemplate } from '@brief/domain/org-template';
 import { BriefAdapter, BriefView } from '@brief/infrastructure/brief.adapter';
+import { LetterPreviewAdapter } from '@brief/infrastructure/letter-preview.adapter';
 
 /** Transient action state (submit/approve/reject/send/resetDemo) — one tagged union
     instead of a busy boolean + a nullable error sitting side by side. */
@@ -35,6 +36,7 @@ type LoadedBriefState = Extract<BriefState, { tag: 'loaded' }>;
 @Injectable({ providedIn: 'root' })
 export class BriefStore {
   private adapter = inject(BriefAdapter);
+  private previewAdapter = inject(LetterPreviewAdapter);
   private store = createStore<BriefState, BriefMsg>(initial, reduce);
 
   readonly model = this.store.model;
@@ -147,6 +149,20 @@ export class BriefStore {
   reject = (comments: string) => this.transition(() => this.adapter.reject(comments));
   send = () => this.transition(() => this.adapter.send());
 
+  /** Explicit action, never a live re-render (PRD §8): opens the server-composed
+      letter in a new tab. ponytail: the blob URL is never revoked — it's cheap and
+      the tab outlives this call; not worth a teardown hook for a POC. */
+  async previewLetter() {
+    this.actionState.set({ tag: 'Busy' });
+    const r = await this.previewAdapter.preview();
+    if (!r.ok) {
+      this.actionState.set({ tag: 'Failed', error: r.error });
+      return;
+    }
+    this.actionState.set({ tag: 'Idle' });
+    window.open(URL.createObjectURL(r.value), '_blank');
+  }
+
   // A transition: flush any pending save, call the server (authoritative), then mirror
   // the returned status through the pure reducer's guarded transition.
   private async transition(action: () => Promise<Result<string, BriefView>>) {