Feat: Add Copy URL button and reusable toast notification system

Detail page now has a "Copy URL" button that copies the image's direct
file URL to the clipboard. A toast service (BehaviorSubject-backed,
auto-dismissing after 3s) confirms success or failure. ToastComponent
is registered at the app root and available to all future features.

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

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

@@ -83,6 +83,14 @@ describe('AppComponent', () => {
     expect(link.getAttribute('href')).toBe('/');
   });
 
+  it('renders app-toast element', () => {
+    authSpy.isAuthenticated.and.returnValue(false);
+    const fixture = TestBed.createComponent(AppComponent);
+    fixture.detectChanges();
+    const toast = (fixture.nativeElement as HTMLElement).querySelector('app-toast');
+    expect(toast).not.toBeNull();
+  });
+
   it('header height is 48px', () => {
     authSpy.isAuthenticated.and.returnValue(true);
     const fixture = TestBed.createComponent(AppComponent);