fix(pdf): merge setElements and render effects so canvas remount triggers re-render

The refactor made pdfDoc a plain variable so renderer.isLoaded was not
reactive. Svelte only tracked currentPage and scale — but when the canvas
reappeared after loading, neither changed, so the PDF stayed blank.

Fix: merge the two effects into one that reads canvasEl synchronously.
Svelte now tracks canvasEl as a dependency; when the canvas remounts
(loading spinner → false), the effect re-fires and renders the
already-loaded PDF document.

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

frontend/src/lib/components/PdfViewer.svelte

@@ -56,21 +56,20 @@ onMount(async () => {
 	await renderer.init();
 });
 
-// Wire DOM elements to the renderer after they mount
-$effect(() => {
-	if (canvasEl && textLayerEl) {
-		renderer.setElements(canvasEl, textLayerEl);
-	}
-});
-
 $effect(() => {
 	if (renderer.pdfjsReady && url) {
 		renderer.loadDocument(url);
 	}
 });
 
+// Wire DOM elements to the renderer and trigger rendering.
+// canvasEl is read synchronously so Svelte tracks it as a dependency:
+// when the canvas reappears after the loading spinner (loading → false),
+// this effect re-fires and renders the already-loaded PDF.
 $effect(() => {
-	// Read scale and currentPage synchronously so Svelte tracks them as dependencies.
+	if (!canvasEl || !textLayerEl) return;
+	renderer.setElements(canvasEl, textLayerEl);
+	// Also track currentPage and scale so page-nav / zoom re-renders work.
 	if (renderer.isLoaded && renderer.currentPage && renderer.scale > 0) {
 		renderer.renderCurrentPage().then(() => renderer.prerender());
 	}