fixed unlinked categories validation

code/frontend/src/app/settings/download-cleaner/download-cleaner-settings.component.html

@@ -248,6 +248,7 @@
                   >
                   </p-autocomplete>
                 </div>
+                <small *ngIf="hasUnlinkedCategoriesError()" class="p-error">At least one category is required when unlinked download handling is enabled</small>
                 <small class="form-helper-text">Categories to check for unlinked downloads</small>
               </div>
             </div>

code/frontend/src/app/settings/download-cleaner/download-cleaner-settings.component.ts

@@ -139,7 +139,7 @@ export class DownloadCleanerSettingsComponent implements OnDestroy, CanComponent
       unlinkedUseTag: [{ value: false, disabled: true }],
       unlinkedIgnoredRootDir: [{ value: '', disabled: true }],
       unlinkedCategories: [{ value: [], disabled: true }]
-    });
+    }, { validators: this.validateUnlinkedCategories });
 
     // Load the current configuration
     effect(() => {
@@ -199,6 +199,20 @@ export class DownloadCleanerSettingsComponent implements OnDestroy, CanComponent
     return null;
   }
   
+  /**
+   * Custom validator for unlinked categories - requires categories when unlinked handling is enabled
+   */
+  private validateUnlinkedCategories(group: FormGroup): ValidationErrors | null {
+    const unlinkedEnabled = group.get('unlinkedEnabled')?.value;
+    const unlinkedCategories = group.get('unlinkedCategories')?.value;
+
+    if (unlinkedEnabled && (!unlinkedCategories || unlinkedCategories.length === 0)) {
+      return { unlinkedCategoriesRequired: true };
+    }
+
+    return null;
+  }
+  
   /**
    * Helper method to get a category control as FormGroup for the template
    */
@@ -592,6 +606,13 @@ export class DownloadCleanerSettingsComponent implements OnDestroy, CanComponent
     return control ? control.touched && control.hasError(errorName) : false;
   }
   
+  /**
+   * Check if the form has the unlinked categories validation error
+   */
+  hasUnlinkedCategoriesError(): boolean {
+    return this.downloadCleanerForm.touched && this.downloadCleanerForm.hasError('unlinkedCategoriesRequired');
+  }
+  
   /**
    * Get schedule value options based on the current schedule unit type
    */