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/quickstart.md

@@ -0,0 +1,33 @@
+# Quickstart: Copy URL & Toast Notifications
+
+## Happy Path — Copy URL
+
+1. Open any image detail page (e.g. `http://localhost:4200/images/{id}`).
+2. Confirm a "Copy URL" button is visible.
+3. Click "Copy URL".
+4. Confirm a success toast appears ("URL copied!" or similar) and then disappears automatically.
+5. Paste into a text editor — confirm the pasted value is the full image file URL.
+
+## Happy Path — Toast Auto-Dismiss
+
+1. Click "Copy URL".
+2. Confirm the toast appears.
+3. Do not interact — wait ~3 seconds.
+4. Confirm the toast disappears on its own.
+
+## Edge Case — Clipboard Unavailable
+
+1. In Firefox, navigate to `about:config` and set `dom.events.asyncClipboard.clipboardItem` to `false` (or test with a non-HTTPS localhost where clipboard API may be blocked).
+2. Click "Copy URL".
+3. Confirm an error toast appears (e.g. "Failed to copy URL") and auto-dismisses.
+
+## Edge Case — Rapid Clicks
+
+1. Click "Copy URL" three times quickly.
+2. Confirm only one toast is visible at a time (new toast replaces old, no overlapping stack).
+
+## Regression — Other Pages
+
+1. Navigate to the library (`/`), upload page (`/upload`), tags page (`/tags`).
+2. Confirm no toast or copy button is visible on these pages.
+3. Confirm existing functionality is unaffected.