fix: no error feedback on failed save/delete/role-change, editors navigated away before save result was known

Products/Categories saveDraft() and deleteOne(), and Users
setRole()/setStatus()/invite(), had no error handler at all - a
failed mutation was completely silent.

Also found and fixed the same premature-navigation bug as the Phase 1
cart fix: both product and category editor pages called
router.navigate() immediately after facade.saveDraft(), before the
save had resolved - so even after adding error feedback, the user
would already be gone from the page before it could show. saveDraft()
now takes an onSuccess callback and only the page navigates on actual
success; on failure it stays put and shows a themed error dialog.

Added a mutationError signal to all three facades and a themed
app-dialog error alert on the products/categories editor + list pages
and the users page.

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

src/app/features/admin/users/facade/admin-users.facade.ts

@@ -12,6 +12,7 @@ export class AdminUsersFacade {
   readonly invitations = signal<AdminInvitation[]>([]);
   readonly loading = signal(false);
   readonly error = signal(false);
+  readonly mutationError = signal<string | null>(null);
   readonly sessionsTarget = signal<AdminUser | null>(null);
   readonly sessions = signal<AdminSession[]>([]);
   readonly auditTarget = signal<AdminUser | null>(null);
@@ -33,16 +34,28 @@ export class AdminUsersFacade {
   }
 
   setRole(userId: string, roleId: string): void {
-    this.gateway.setUserRole(userId, roleId).pipe(take(1)).subscribe({ next: () => this.loadAll() });
+    this.mutationError.set(null);
+    this.gateway.setUserRole(userId, roleId).pipe(take(1)).subscribe({
+      next: () => this.loadAll(),
+      error: () => this.mutationError.set('common.errorDescription')
+    });
   }
 
   setStatus(userId: string, status: AdminUserStatus): void {
-    this.gateway.setUserStatus(userId, status).pipe(take(1)).subscribe({ next: () => this.loadAll() });
+    this.mutationError.set(null);
+    this.gateway.setUserStatus(userId, status).pipe(take(1)).subscribe({
+      next: () => this.loadAll(),
+      error: () => this.mutationError.set('common.errorDescription')
+    });
   }
 
   invite(email: string, roleId: string, scope: AdminUserScope): void {
     if (!email.trim()) return;
-    this.gateway.inviteUser(email.trim(), roleId, scope).pipe(take(1)).subscribe({ next: () => this.loadAll() });
+    this.mutationError.set(null);
+    this.gateway.inviteUser(email.trim(), roleId, scope).pipe(take(1)).subscribe({
+      next: () => this.loadAll(),
+      error: () => this.mutationError.set('common.errorDescription')
+    });
   }
 
   revokeInvitation(id: string): void {