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>

specs/016-copy-url-toast/contracts/toast-service.md

@@ -0,0 +1,50 @@
+# Contract: ToastService
+
+**Location**: `ui/src/app/services/toast.service.ts`
+**Provided in**: `root` (singleton)
+
+## Interface
+
+```typescript
+interface Toast {
+  message: string;
+  type: 'success' | 'error';
+}
+
+class ToastService {
+  // Observable — emits a Toast when one is active, null when none.
+  readonly current$: Observable<Toast | null>;
+
+  // Show a toast. Replaces any currently-visible toast.
+  // duration defaults to 3000ms.
+  show(message: string, type?: 'success' | 'error', duration?: number): void;
+}
+```
+
+## Behaviour
+
+- `show()` emits the toast immediately on `current$`.
+- After `duration` ms, emits `null` to dismiss.
+- Calling `show()` again before the timer expires resets the timer (new toast replaces old).
+- `type` defaults to `'success'`.
+- `duration` defaults to `3000`.
+
+## Usage Example
+
+```typescript
+// In any component:
+constructor(private toast: ToastService) {}
+
+async copyUrl() {
+  try {
+    await navigator.clipboard.writeText(url);
+    this.toast.show('URL copied!');
+  } catch {
+    this.toast.show('Failed to copy URL', 'error');
+  }
+}
+```
+
+## Consumer: ToastComponent
+
+`ToastComponent` subscribes to `current$` via the `async` pipe and renders/hides based on the emitted value. It is placed once in `AppComponent` and is always present in the DOM.