feat(ownership): add take-ownership preflight endpoint

Enables dry-run checking before committing to case adoption. The preflight
shares the same side-effect-free checks (steps 1–3) as the real take-ownership
handler, so it cannot drift from what will actually succeed. Returns the same
status codes and error shapes as the real endpoint (200 with wouldSucceed:true,
or 409/404/422 if it would fail).

Portal renders a "Vooraf controleren" button for legacy cases, surfaced through
the existing actions block pattern. Confirmed in smoke.sh with two cases: one
where preflight predicts success (and writes nothing), one where it predicts
a named invariant failure (matching what the real call reproduces).

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

portal-frontend/src/app/case-detail/edit-applicant-details/edit-applicant-details.html

@@ -1,12 +1,12 @@
 <form (ngSubmit)="submit()">
   @if (isWriteThrough) {
-    <p class="disclaimer" data-testid="write-through-disclaimer">
+    <p class="banner banner--warning" data-testid="write-through-disclaimer">
       Gevalideerd door het legacy systeem — Angular voegt hier geen eigen validatie toe.
     </p>
   }
 
   @if (banner) {
-    <p class="banner" data-testid="edit-details-banner">{{ banner }}</p>
+    <p class="banner banner--error" data-testid="edit-details-banner">{{ banner }}</p>
   }
 
   <label>
@@ -69,5 +69,7 @@
     <input type="text" name="preferredChannel" required [(ngModel)]="preferredChannel" />
   </label>
 
-  <button type="submit">Opslaan</button>
+  <button type="submit" class="btn btn--primary" data-testid="save-details-submit" [disabled]="saving()">
+    {{ saving() ? 'Bezig met opslaan…' : 'Opslaan' }}
+  </button>
 </form>