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/toast/toast.component.ts

@@ -0,0 +1,44 @@
+import { Component, ChangeDetectionStrategy } from '@angular/core';
+import { CommonModule } from '@angular/common';
+import { ToastService } from '../services/toast.service';
+
+@Component({
+  selector: 'app-toast',
+  standalone: true,
+  imports: [CommonModule],
+  changeDetection: ChangeDetectionStrategy.OnPush,
+  template: `
+    <div
+      *ngIf="toastService.current$ | async as toast"
+      class="toast"
+      [class.success]="toast.type === 'success'"
+      [class.error]="toast.type === 'error'"
+    >{{ toast.message }}</div>
+  `,
+  styles: [`
+    .toast {
+      position: fixed;
+      bottom: 24px;
+      left: 50%;
+      transform: translateX(-50%);
+      padding: 10px 20px;
+      border-radius: var(--radius);
+      font-size: 0.9rem;
+      pointer-events: none;
+      z-index: 1000;
+      white-space: nowrap;
+    }
+    .success {
+      background: var(--surface-raised);
+      color: var(--text);
+      border: 1px solid var(--border);
+    }
+    .error {
+      background: var(--danger);
+      color: var(--danger-text);
+    }
+  `],
+})
+export class ToastComponent {
+  constructor(public toastService: ToastService) {}
+}