feat(a11y): respect prefers-reduced-motion for scroll-sync

Uses scrollIntoView behavior 'instant' instead of 'smooth', skips
CSS animations (static highlight instead), and extends timeout to
2s for reduced-motion users.

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

frontend/src/lib/components/AnnotationLayer.svelte

@@ -194,4 +194,11 @@ const containerStyle = $derived(
 .annotation-flash {
 	animation: annotation-flash-anim 1.5s ease-out;
 }
+
+@media (prefers-reduced-motion: reduce) {
+	.annotation-flash {
+		animation: none;
+		outline: 3px solid rgba(0, 199, 177, 0.8);
+	}
+}
 </style>