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.ts

@@ -1,5 +1,5 @@
 import { HttpErrorResponse } from '@angular/common/http';
-import { Component, EventEmitter, Input, OnInit, Output, inject } from '@angular/core';
+import { Component, EventEmitter, Input, OnInit, Output, inject, signal } from '@angular/core';
 import { FormsModule } from '@angular/forms';
 
 import { ErrorsResponse, InvariantViolationResponse, MessageResponse } from '../../shared/api-error.types';
@@ -32,6 +32,7 @@ export class EditApplicantDetails implements OnInit {
 
   fieldErrors: Record<string, string> = {};
   banner: string | null = null;
+  readonly saving = signal(false);
 
   get isWriteThrough(): boolean {
     return this.actionLink.mode === 'writeThrough';
@@ -52,6 +53,7 @@ export class EditApplicantDetails implements OnInit {
   submit(): void {
     this.fieldErrors = {};
     this.banner = null;
+    this.saving.set(true);
 
     const body: ApplicantDetailsRequest = {
       surname: this.surname,
@@ -63,8 +65,14 @@ export class EditApplicantDetails implements OnInit {
     };
 
     this.caseDetailService.updateDetails(this.actionLink.href, body).subscribe({
-      next: () => this.saved.emit(),
-      error: (error: HttpErrorResponse) => this.handleError(error),
+      next: () => {
+        this.saving.set(false);
+        this.saved.emit();
+      },
+      error: (error: HttpErrorResponse) => {
+        this.saving.set(false);
+        this.handleError(error);
+      },
     });
   }