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/services/toast.service.ts

@@ -0,0 +1,25 @@
+import { Injectable } from '@angular/core';
+import { BehaviorSubject, Observable } from 'rxjs';
+
+export interface Toast {
+  message: string;
+  type: 'success' | 'error';
+}
+
+@Injectable({ providedIn: 'root' })
+export class ToastService {
+  private readonly subject = new BehaviorSubject<Toast | null>(null);
+  readonly current$: Observable<Toast | null> = this.subject.asObservable();
+  private timer: ReturnType<typeof setTimeout> | null = null;
+
+  show(message: string, type: 'success' | 'error' = 'success', duration = 3000): void {
+    if (this.timer !== null) {
+      clearTimeout(this.timer);
+    }
+    this.subject.next({ message, type });
+    this.timer = setTimeout(() => {
+      this.subject.next(null);
+      this.timer = null;
+    }, duration);
+  }
+}