feat(portal): real diploma file upload on the self-service page (refs #103)

Replaces the S-10a stub button with a labelled file input (accept application/pdf);
the component base64-encodes the chosen file client-side and posts it (with its name
and type) keyed by the reference, confirming on success and surfacing a retryable
failure. The upload button stays disabled until a file is chosen.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/self-service/src/app/registration/registration-page.html

@@ -19,11 +19,20 @@
               Het aanleveren van uw documenten is niet gelukt. Probeer het opnieuw.
             </p>
           }
+          <p utrecht-paragraph>Lever uw diploma aan (PDF).</p>
+          <label utrecht-form-label for="diploma">Diploma</label>
+          <input
+            id="diploma"
+            type="file"
+            accept="application/pdf"
+            [disabled]="providingDocuments()"
+            (change)="onFileSelected($event)"
+          />
           <button
             utrecht-button
             appearance="primary-action-button"
             type="button"
-            [disabled]="providingDocuments()"
+            [disabled]="providingDocuments() || !selectedFile()"
             (click)="provideDocuments()"
           >
             Documenten aanleveren