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/library/library.component.ts

@@ -21,7 +21,7 @@ const PLACEHOLDER_SVG = `data:image/svg+xml,<svg xmlns="http://www.w3.org/2000/s
   template: `
     <div class="library">
       <header>
-        <h1><a class="home-link" (click)="router.navigate(['/'])">Reactbin</a></h1>
+        <h1><a class="home-link" routerLink="/" [queryParams]="{}">Reactbin</a></h1>
         <div class="header-actions">
           <a routerLink="/tags" class="tags-link">Browse tags</a>
           <button class="upload-btn" (click)="router.navigate(['/upload'])">Upload</button>