Registratie: answer-driven required document uploads

Categories stay server-owned (ADR-0001); the FE sends its answers to
/uploads/categories and re-fetches reactively when they change:
- Diplomabewijs required only for a handmatig diploma (DUO is verified digitally;
  nothing required before a diploma is chosen).
- Bewijs Nederlandse taalvaardigheid required only when the applicant answers "ja"
  to the nl-taalvaardigheid (B2) policy question.
CategoriesFor(wizardId, diplomaHerkomst, taalvaardigheid) decides; Find uses the
maximal set so uploads still validate. CategoriesLoaded drops orphaned uploads
when a category disappears. Also: show the foreground-only upload banner only when
there is at least one category.

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

src/app/shared/ui/upload/document-upload/document-upload.component.ts

@@ -16,7 +16,7 @@ import { UploadStatusBannerComponent } from '../upload-status-banner/upload-stat
     @if (state().categoriesError) {
       <app-upload-status-banner type="error" [message]="state().categoriesError!" />
     } @else {
-      @if (state().backgroundSyncAvailable === false) {
+      @if (state().backgroundSyncAvailable === false && state().categories.length > 0) {
         <app-upload-status-banner type="info" [message]="foregroundOnlyMessage" />
       }
       @for (c of state().categories; track c.categoryId) {