Fix: Resolve 13 pre-existing UI test failures across Login, Upload, and Detail components

- LoginComponent: provide ActivatedRoute stub (component reads returnUrl query param)
- UploadComponent: add cdr.markForCheck() to handleUploadError so OnPush view updates
  when the method is called directly; fix success test to check showSuccess not toastMessage
- DetailComponent: drive not-found-card and tag-error tests through component methods
  that call markForCheck() rather than directly mutating state on OnPush components

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ui/src/app/detail/detail.component.spec.ts

@@ -2,7 +2,7 @@ import { TestBed } from '@angular/core/testing';
 import { ActivatedRoute, provideRouter, Router } from '@angular/router';
 import { provideHttpClient } from '@angular/common/http';
 import { provideHttpClientTesting } from '@angular/common/http/testing';
-import { of, throwError, Subject } from 'rxjs';
+import { of, throwError, Subject, NEVER } from 'rxjs';
 import { DetailComponent } from './detail.component';
 import { ImageService } from '../services/image.service';
 import { ToastService } from '../services/toast.service';
@@ -113,17 +113,15 @@ describe('DetailComponent', () => {
   });
 
   it('not-found card shown when image is null, loading is false, error is false', () => {
-    const { fixture, component } = setup('img-1', of(MOCK_IMAGE));
-    component.image = null;
-    component.loading = false;
-    component.error = false;
-    fixture.detectChanges();
+    // Service returns null → fetchImage sets image=null, loading=false, markForCheck()
+    const { fixture } = setup('img-1', of(null as any));
     expect((fixture.nativeElement as HTMLElement).querySelector('.not-found-card')).not.toBeNull();
   });
 
   it('tag error element uses danger styling class', () => {
-    const { fixture, component } = setup();
-    component.tagError = 'Invalid tag: special characters not allowed';
+    const { fixture, component, imgSvc } = setup();
+    spyOn(imgSvc, 'updateTags').and.returnValue(throwError(() => ({ error: { detail: 'Invalid tag' } })));
+    component.addTag('bad#tag');
     fixture.detectChanges();
     const errEl = (fixture.nativeElement as HTMLElement).querySelector('.tag-error');
     expect(errEl).not.toBeNull();