fix(frontend): zoom buttons now re-render immediately (#39)

scale was only read inside the async renderPage function, so Svelte 5
never tracked it as a reactive dependency of the effect. Reading scale
synchronously in the effect condition registers it as a dependency and
triggers a re-render on every zoom change.

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

frontend/src/lib/components/PdfViewer.svelte

@@ -141,7 +141,10 @@ $effect(() => {
 });
 
 $effect(() => {
-	if (pdfDoc && currentPage) {
+	// Read scale synchronously so Svelte tracks it as a dependency.
+	// Without this, zoom changes don't re-trigger the effect because
+	// scale is only read inside the async renderPage call.
+	if (pdfDoc && currentPage && scale > 0) {
 		renderPage(pdfDoc, currentPage).then(() => {
 			if (pdfDoc) prerender(pdfDoc, currentPage);
 		});