fix(learner): destroy decks through the pdf.js loading task

pdf.js 6 documents have no destroy(); leaving a lesson whose deck had loaded threw in the effect cleanup and
unmounted the app.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

packages/feature-learner/src/web/lesson/DeckViewer.tsx

@@ -49,17 +49,21 @@ export function DeckViewer({ deck, notes, initialPage = 1, keyboard = true, titl
   const canvas = useRef<HTMLCanvasElement>(null);
 
   useEffect(() => {
-    let live = true, loaded: PdfDoc | null = null;
+    // pdf.js 6 documents have no destroy(); the loading task owns the document and the worker port.
+    let live = true, task: { destroy(): Promise<void> } | null = null;
     setDoc(null); setError(null); setPages(0);
     loadPdfjs()
-      .then((pdfjs) => pdfjs.getDocument({ url: deck.url, withCredentials: true }).promise)
+      .then((pdfjs) => {
+        const t = pdfjs.getDocument({ url: deck.url, withCredentials: true });
+        task = t;
+        return t.promise;
+      })
       .then((d: PdfDoc) => {
-        loaded = d;
-        if (!live) { void d.destroy(); return; }
+        if (!live) return;
         setDoc(d); setPages(d.numPages); setPage((p) => Math.min(Math.max(1, p), d.numPages));
       })
       .catch((e: unknown) => live && setError(`This deck could not be opened. ${(e as Error).message ?? ""}`.trim()));
-    return () => { live = false; if (loaded) void loaded.destroy(); };
+    return () => { live = false; void task?.destroy().catch(() => {}); };
   }, [deck.url]);
 
   // Render the current page to fit the stage.