Администратор открывает любой ящик: MAILBOXES → OPEN, полоса «вернуться в admin@»

POST /api/admin/mailboxes/{id}/open — сессия этого ящика без его пароля, только для
MAIL_ADMINS; каждое открытие — в лог (кто какой ящик открыл). В веб-почте: кнопка OPEN
у ящика, сверху розовая полоса «Вы в ящике X как администратор — Back to admin@».
Тесты: +1 (всего 75), ng build OK.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/app/pages/settings/mailboxes.section.ts

@@ -5,6 +5,7 @@ import { ApiService } from '../../core/api.service';
 import { MailStateService } from '../../core/mail-state.service';
 import { Mailbox, errorText, formatBytes } from '../../core/models';
 import { ToastService } from '../../ui/toast.service';
+import { AuthService } from '../../core/auth.service';
 
 /** Settings → Mailboxes (administrators): create no-reply@, monitor@, support@ …, new password, switch off. */
 @Component({
@@ -46,6 +47,10 @@ import { ToastService } from '../../ui/toast.service';
             <span class="text-sm text-slate-400">{{ m.displayName }}</span>
             @if (m.admin) { <span class="text-[10px] font-mono px-1.5 rounded border border-pink-500/40 text-pink-300">ADMIN</span> }
             <span class="text-xs font-mono text-slate-500 ml-auto">{{ size(m.usedBytes) }}</span>
+            @if (m.email !== me()) {
+              <button type="button" class="ghost-btn !py-1" (click)="open(m)" [attr.aria-label]="'Open ' + m.email" title="Open mailbox">
+                <mat-icon>mark_email_read</mat-icon> OPEN</button>
+            }
             <button type="button" class="ghost-btn !py-1" (click)="newPassword(m)" [attr.aria-label]="'New password for ' + m.email"><mat-icon>key</mat-icon></button>
             @if (m.email !== me()) {
               <button type="button" class="ghost-btn !py-1" (click)="toggle(m)"
@@ -62,6 +67,7 @@ export class MailboxesSection {
   private api = inject(ApiService);
   private state = inject(MailStateService);
   private toasts = inject(ToastService);
+  private auth = inject(AuthService);
 
   readonly mailboxes = signal<Mailbox[]>([]);
   readonly secret = signal<{ email: string; password: string } | null>(null);
@@ -111,6 +117,14 @@ export class MailboxesSection {
     });
   }
 
+  /** Administrator reads this mailbox: its session; the banner on top leads back. Full reload — fresh folders. */
+  open(m: Mailbox): void {
+    this.api.openMailbox(m.id).subscribe({
+      next: session => { this.auth.openAs(session); location.assign('/'); },
+      error: e => this.toasts.error(errorText(e))
+    });
+  }
+
   toggle(m: Mailbox): void {
     this.api.setMailboxActive(m.id, !m.active).subscribe({
       next: () => this.load(),