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/PdfViewer.svelte

@@ -17,7 +17,8 @@ let {
 	onAnnotationClick,
 	onTranscriptionDraw,
 	documentFileHash,
-	annotationsDimmed = false
+	annotationsDimmed = false,
+	flashAnnotationId = null
 }: {
 	url: string;
 	documentId?: string;
@@ -29,6 +30,7 @@ let {
 	onTranscriptionDraw?: (rect: DrawRect) => void;
 	documentFileHash?: string | null;
 	annotationsDimmed?: boolean;
+	flashAnnotationId?: string | null;
 } = $props();
 
 let pdfDoc = $state<PDFDocumentProxy | null>(null);
@@ -459,6 +461,7 @@ function zoomOut() {
 								blockNumbers={blockNumbers}
 								activeAnnotationId={activeAnnotationId}
 								dimmed={annotationsDimmed}
+								flashAnnotationId={flashAnnotationId}
 								onDraw={handleDraw}
 								onAnnotationClick={handleAnnotationClick}
 							/>