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/ui/brief.page.ts

@@ -63,6 +63,7 @@ import { LetterComposerComponent } from '@brief/ui/letter-composer/letter-compos
                 (approve)="store.approve()"
                 (reject)="store.reject($event)"
                 (send)="store.send()"
+                (preview)="store.previewLetter()"
               />
             }
           }

src/app/brief/ui/letter-composer/letter-composer.component.ts

@@ -41,6 +41,11 @@ import { RejectionCommentsComponent } from '@brief/ui/rejection-comments/rejecti
         flex-wrap: wrap;
         margin-block-end: var(--rhc-space-max-lg);
       }
+      .head-end {
+        display: flex;
+        align-items: center;
+        gap: var(--rhc-space-max-md);
+      }
       .panel {
         margin-block: var(--rhc-space-max-xl);
       }
@@ -56,7 +61,12 @@ import { RejectionCommentsComponent } from '@brief/ui/rejection-comments/rejecti
   template: `
     <div class="head">
       <app-heading [level]="2">{{ title() }}</app-heading>
-      <app-status-badge [label]="statusLabel()" [color]="statusColor()" />
+      <div class="head-end">
+        <app-button variant="subtle" [disabled]="busy()" (click)="preview.emit()">{{
+          previewLabel()
+        }}</app-button>
+        <app-status-badge [label]="statusLabel()" [color]="statusColor()" />
+      </div>
     </div>
 
     @if (status() === 'rejected') {
@@ -143,9 +153,11 @@ export class LetterComposerComponent {
   approve = output<void>();
   reject = output<string>();
   send = output<void>();
+  preview = output<void>();
   locate = output<Diagnostic>();
 
   title = input($localize`:@@brief.title:Brief aan de zorgverlener`);
+  previewLabel = input($localize`:@@brief.preview.open:Voorbeeld`);
   submitLabel = input($localize`:@@brief.submit:Indienen ter beoordeling`);
   resubmitLabel = input($localize`:@@brief.resubmit:Opnieuw indienen`);
   submitHint = input(