Fix the Malware Blocker page not displaying validation errors (#473)

code/frontend/src/app/core/interceptors/error.interceptor.ts

@@ -14,8 +14,11 @@ export const errorInterceptor: HttpInterceptorFn = (req, next) => {
       if (error.error instanceof ErrorEvent) {
         // Client-side error
         message = error.error.message;
+      } else if (typeof error.error === 'string') {
+        // Server-side error with plain string body
+        message = error.error;
       } else {
-        // Server-side error
+        // Server-side error with JSON body
         message = error.error?.error
           ?? error.error?.message
           ?? error.message

code/frontend/src/app/features/settings/malware-blocker/malware-blocker.component.html

@@ -70,7 +70,7 @@
   </app-card>
 
   @if (enabled()) {
-    <app-accordion header="Arr Blocklists" subtitle="Per-application blocklist configuration" [(expanded)]="arrExpanded">
+    <app-accordion header="Arr Blocklists" subtitle="Per-application blocklist configuration" [(expanded)]="arrExpanded" [error]="noBlocklistError()">
       @for (name of arrNames; track name) {
         <div class="arr-blocklist">
           <h4 class="arr-blocklist__title">{{ capitalize(name) }}</h4>

code/frontend/src/app/features/settings/malware-blocker/malware-blocker.component.ts

@@ -6,6 +6,7 @@ import {
   type SelectOption,
 } from '@ui';
 import { MalwareBlockerApi } from '@core/api/malware-blocker.api';
+import { ApiError } from '@core/interceptors/error.interceptor';
 import { ToastService } from '@core/services/toast.service';
 import { MalwareBlockerConfig, BlocklistSettings, MalwareScheduleOptions } from '@shared/models/malware-blocker-config.model';
 import { BlocklistType, ScheduleUnit } from '@shared/models/enums';
@@ -120,7 +121,18 @@ export class MalwareBlockerComponent implements OnInit, HasPendingChanges {
     return undefined;
   }
 
+  readonly noBlocklistError = computed(() => {
+    if (!this.enabled()) return undefined;
+    const blocklists = this.arrBlocklists();
+    const hasAnyEnabled = ARR_NAMES.some(name => blocklists[name]?.enabled);
+    if (!hasAnyEnabled) {
+      return 'At least one blocklist must be configured';
+    }
+    return undefined;
+  });
+
   readonly hasErrors = computed(() => {
+    if (this.noBlocklistError()) return true;
     if (this.scheduleEveryError()) return true;
     if (this.cronError()) return true;
     if (this.chipInputs().some(c => c.hasUncommittedInput())) return true;
@@ -225,8 +237,10 @@ export class MalwareBlockerComponent implements OnInit, HasPendingChanges {
         setTimeout(() => this.saved.set(false), 1500);
         this.savedSnapshot.set(this.buildSnapshot());
       },
-      error: () => {
-        this.toast.error('Failed to save malware blocker settings');
+      error: (err: ApiError) => {
+        this.toast.error(err.statusCode === 400
+          ? err.message
+          : 'Failed to save malware blocker settings');
         this.saving.set(false);
       },
     });