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/upload/upload.machine.spec.ts

@@ -44,6 +44,15 @@ describe('CategoriesLoaded', () => {
     const s = reduceUpload({ ...initialUpload, categoriesError: 'boom' }, { type: 'CategoriesLoaded', categories: [] });
     expect(s.categoriesError).toBeUndefined();
   });
+
+  it('drops uploads + channel choices for categories that disappear', () => {
+    const s0 = select(stateWith([cat({ categoryId: 'diploma' }), cat({ categoryId: 'id' })], { deliveryChannel: { id: 'post' } }), 'diploma', 'u1');
+    expect(get(s0, 'u1')).toBeDefined();
+    // Reload with the diploma category gone (e.g. DUO chosen).
+    const s1 = reduceUpload(s0, { type: 'CategoriesLoaded', categories: [cat({ categoryId: 'id' })] });
+    expect(get(s1, 'u1')).toBeUndefined();
+    expect(s1.deliveryChannel).toEqual({ id: 'post' });
+  });
 });
 
 describe('CategoriesLoadFailed / BackgroundSyncAvailability', () => {