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/DocumentViewer.svelte

@@ -21,6 +21,7 @@ type Props = {
 	annotationReloadKey?: number;
 	activeAnnotationId: string | null;
 	annotationsDimmed?: boolean;
+	flashAnnotationId?: string | null;
 	onAnnotationClick: (id: string) => void;
 	onTranscriptionDraw?: (rect: DrawRect) => void;
 };
@@ -35,6 +36,7 @@ let {
 	annotationReloadKey = 0,
 	activeAnnotationId = $bindable(),
 	annotationsDimmed = false,
+	flashAnnotationId = null,
 	onAnnotationClick,
 	onTranscriptionDraw
 }: Props = $props();
@@ -93,6 +95,7 @@ let {
 			annotationReloadKey={annotationReloadKey}
 			bind:activeAnnotationId={activeAnnotationId}
 			annotationsDimmed={annotationsDimmed}
+			flashAnnotationId={flashAnnotationId}
 			onAnnotationClick={onAnnotationClick}
 			onTranscriptionDraw={onTranscriptionDraw}
 			documentFileHash={doc.fileHash ?? null}