fix(pdf): prevent scroll-sync effect from hijacking page navigation

The scroll-sync $effect was re-triggering on every dependency change
(including currentPage), forcing the PDF back to the annotation's page
when the user clicked next/prev. Fix: track prevActiveAnnotationId
and only scroll when the active annotation actually changes.

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

frontend/src/lib/components/PdfViewer.svelte

@@ -218,9 +218,16 @@ $effect(() => {
 });
 
 // Scroll-sync: when activeAnnotationId changes, navigate to its page
+let prevActiveAnnotationId: string | null = null;
 $effect(() => {
-	if (!activeAnnotationId || !pdfDoc) return;
+	const id = activeAnnotationId;
-	const ann = annotations.find((a) => a.id === activeAnnotationId);
+	if (!id || id === prevActiveAnnotationId || !pdfDoc) {
+		prevActiveAnnotationId = id;
+		return;
+	}
+	prevActiveAnnotationId = id;
+
+	const ann = annotations.find((a) => a.id === id);
 	if (!ann) return;
 
 	if (ann.pageNumber !== currentPage) {
@@ -228,10 +235,9 @@ $effect(() => {
 	}
 
 	// After page renders, scroll the annotation into view (double-rAF for async render)
-	const targetId = activeAnnotationId;
 	requestAnimationFrame(() => {
 		requestAnimationFrame(() => {
-			const el = document.querySelector(`[data-testid="annotation-${targetId}"]`);
+			const el = document.querySelector(`[data-testid="annotation-${id}"]`);
 			el?.scrollIntoView({ behavior: 'smooth', block: 'center' });
 		});
 	});