fix(flags): surface a failed admin toggle instead of swallowing it

FeatureFlagStore.set() was try/finally with no catch. A rejected
PUT /admin/flags/{key} escaped into the `void this.store.set(...)` call site
as an unhandled promise rejection; the finally-block reload then snapped the
control back to its old value. The admin saw a toggle that silently refused
to move, with no error rendered anywhere and nothing in the state.

set() now folds through the existing runSubmit helper and returns
Result<string, void>, reloading either way so the state still reflects the
server. The page awaits it and renders the failure in an app-alert.

Found by the CQRS-light pass (CQ-002/CQ-004) as one of three mutations that
reach the raw ApiClient without producing a Result — the baseline's BL-007
inventory had missed all three.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

libs/beheer/src/ui/feature-flags.page.ts

@@ -1,4 +1,4 @@
-import { Component, computed, inject } from '@angular/core';
+import { Component, computed, inject, signal } from '@angular/core';
 import { PageShellComponent } from '@shared/layout/page-shell/page-shell.component';
 import { AlertComponent } from '@shared/ui/alert/alert.component';
 import { ButtonComponent } from '@shared/ui/button/button.component';
@@ -45,6 +45,9 @@ import { FeatureFlagStore } from '@shared/application/feature-flags.store';
       } @else if (!canManage()) {
         <app-alert type="error">{{ deniedText }}</app-alert>
       } @else {
+        @if (setError(); as e) {
+          <app-alert type="error">{{ e }}</app-alert>
+        }
         <app-async [data]="store.flags()">
           <ng-template appAsyncError>
             <app-alert type="error">{{ failedText }}</app-alert>
@@ -89,8 +92,13 @@ export class FeatureFlagsPage {
   protected enableText = $localize`:@@flags.enable:Aanzetten`;
   protected disableText = $localize`:@@flags.disable:Uitzetten`;
 
-  protected toggle(key: string, enabled: boolean) {
-    void this.store.set(key, enabled);
+  /** The last failed toggle, so a rejected PUT is visible instead of silently snapping back. */
+  protected setError = signal<string | null>(null);
+
+  protected async toggle(key: string, enabled: boolean) {
+    this.setError.set(null);
+    const r = await this.store.set(key, enabled);
+    if (!r.ok) this.setError.set(r.error);
   }
   protected reload() {
     void this.store.load();