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

@@ -54,6 +54,7 @@ let transcribeMode = $state(false);
 let panelMode = $state<'read' | 'edit'>('read');
 let activeAnnotationId = $state<string | null>(null);
 let highlightBlockId = $state<string | null>(null);
+let flashAnnotationId = $state<string | null>(null);
 
 // ── Transcription blocks ─────────────────────────────────────────────────────
 
@@ -177,6 +178,10 @@ async function handleAnnotationClick(annotationId: string) {
 
 function handleParagraphClick(annotationId: string) {
 	activeAnnotationId = annotationId;
+	flashAnnotationId = annotationId;
+	setTimeout(() => {
+		flashAnnotationId = null;
+	}, 1500);
 }
 
 // Load blocks when transcribe mode is entered and set default panel mode
@@ -241,6 +246,7 @@ onMount(() => {
 				blockNumbers={blockNumbers}
 				annotationReloadKey={annotationReloadKey}
 				annotationsDimmed={transcribeMode && panelMode === 'read'}
+				flashAnnotationId={flashAnnotationId}
 				bind:activeAnnotationId={activeAnnotationId}
 				onAnnotationClick={handleAnnotationClick}
 				onTranscriptionDraw={createBlockFromDraw}