fix(pdf): guard against null textLayerEl in renderPage

Prevents 'can't access property innerHTML, textDiv is null' when
the component unmounts while a render is in flight (e.g. switching
to OCR progress view tears down the panel content).

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

frontend/src/lib/components/PdfViewer.svelte

@@ -145,6 +145,7 @@ async function renderPage(doc: PDFDocumentProxy, pageNum: number) {
 
 	// Text layer
 	const textDiv = textLayerEl;
+	if (!textDiv) return;
 	textDiv.innerHTML = '';
 	textDiv.style.width = `${viewport.width / dpr}px`;
 	textDiv.style.height = `${viewport.height / dpr}px`;