fix(e2e): fix annotation delete test and harden comments fetch

- Add aria-label="Kommentare anzeigen" to annotation container div so
  getByRole('button', { name: /annotation löschen/i }) no longer
  matches the container (its name was previously inherited from the
  child delete button, causing the test to click the wrong element)
- Wrap the server-side comments fetch in a .catch and try/catch so a
  network error or non-JSON response never crashes the document load

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/components/AnnotationLayer.svelte

@@ -120,6 +120,7 @@ const containerStyle = $derived(
 			data-annotation
 			role="button"
 			tabindex="0"
+			aria-label="Kommentare anzeigen"
 			onclick={() => onAnnotationClick?.(annotation.id)}
 			onkeydown={(e) => { if (e.key === 'Enter' || e.key === ' ') onAnnotationClick?.(annotation.id); }}
 			onmouseenter={() => (hoveredId = annotation.id)}