feat(ui): add bidirectional scroll-sync with flash animations

Paragraph click flashes the PDF annotation outline (1.5s fade).
Annotation click highlights the paragraph with a background flash.
Both directions scroll the target into view.

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

frontend/src/lib/components/AnnotationLayer.svelte

@@ -15,6 +15,7 @@ let {
 	blockNumbers = {},
 	activeAnnotationId = null,
 	dimmed = false,
+	flashAnnotationId = null,
 	onDraw,
 	onAnnotationClick
 }: {
@@ -24,6 +25,7 @@ let {
 	blockNumbers?: Record<string, number>;
 	activeAnnotationId?: string | null;
 	dimmed?: boolean;
+	flashAnnotationId?: string | null;
 	onDraw: (rect: DrawRect) => void;
 	onAnnotationClick?: (id: string) => void;
 } = $props();
@@ -110,6 +112,7 @@ const containerStyle = $derived(
 		<div
 			data-testid="annotation-{annotation.id}"
 			data-annotation
+			class:annotation-flash={flashAnnotationId === annotation.id}
 			role="button"
 			tabindex="0"
 			aria-label="Block anzeigen"
@@ -175,3 +178,20 @@ const containerStyle = $derived(
 		></div>
 	{/if}
 </div>
+
+<style>
+@keyframes annotation-flash-anim {
+	0% {
+		outline: 3px solid rgba(0, 199, 177, 0.8);
+		outline-offset: 0px;
+	}
+	100% {
+		outline: 3px solid rgba(0, 199, 177, 0);
+		outline-offset: 2px;
+	}
+}
+
+.annotation-flash {
+	animation: annotation-flash-anim 1.5s ease-out;
+}
+</style>