docs(pdf-viewer): comment untrack invariant on renderer init

Without untrack, a reactive libLoader prop reference change would
reinitialise the whole renderer and lose all loaded state.

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

frontend/src/lib/document/viewer/PdfViewer.svelte

@@ -39,6 +39,7 @@ let {
 	libLoader?: LibLoader;
 } = $props();
 
+// untrack: libLoader prop change must not reinitialise the renderer
 const renderer = untrack(() => createPdfRenderer(libLoader));
 
 // Canvas and text layer container refs — bound via bind:this