feat(transcription): mobile stacked layout + cross-page scroll-sync

Mobile layout (< 768px):
- Split view stacks vertically: PDF top (min 40vh), blocks below
- Blocks panel gets border-top instead of border-left
- PDF remains interactive for drawing in stacked mode

Scroll-sync (block → PDF):
- Clicking a block sets activeAnnotationId
- PdfViewer effect watches activeAnnotationId, navigates to the
  annotation's page if different from current, then scrolls the
  annotation element into view (double-rAF for async render timing)
- Works across pages: block on page 3 navigates PDF to page 3

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

frontend/src/lib/components/PdfViewer.svelte

@@ -217,6 +217,26 @@ $effect(() => {
 	if (transcribeMode) showAnnotations = true;
 });
 
+// Scroll-sync: when activeAnnotationId changes, navigate to its page
+$effect(() => {
+	if (!activeAnnotationId || !pdfDoc) return;
+	const ann = annotations.find((a) => a.id === activeAnnotationId);
+	if (!ann) return;
+
+	if (ann.pageNumber !== currentPage) {
+		currentPage = ann.pageNumber;
+	}
+
+	// After page renders, scroll the annotation into view (double-rAF for async render)
+	const targetId = activeAnnotationId;
+	requestAnimationFrame(() => {
+		requestAnimationFrame(() => {
+			const el = document.querySelector(`[data-testid="annotation-${targetId}"]`);
+			el?.scrollIntoView({ behavior: 'smooth', block: 'center' });
+		});
+	});
+});
+
 function prevPage() {
 	if (currentPage > 1) currentPage -= 1;
 }