fix(viewer): check res.ok on orphaned annotation DELETE to surface errors

Without the guard, a failed DELETE (4xx/5xx) was silently swallowed and
annotationReloadKey was incremented anyway, leaving the annotation visible
and the user with no feedback. Now matches the deleteBlock() pattern
immediately above.

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

frontend/src/routes/documents/[id]/+page.svelte

@@ -120,7 +120,10 @@ async function handleAnnotationDeleteRequest(annotationId: string) {
 		await deleteBlock(block.id);
 	} else {
 		// Annotation has no linked block — delete the annotation directly
-		await fetch(`/api/documents/${doc.id}/annotations/${annotationId}`, { method: 'DELETE' });
+		const res = await fetch(`/api/documents/${doc.id}/annotations/${annotationId}`, {
+			method: 'DELETE'
+		});
+		if (!res.ok) throw new Error('Delete annotation failed');
 		annotationReloadKey++;
 	}
 }