fix(preview): show render errors instead of a blank pane

The render loop swallowed getDocument/render rejections, leaving the
preview silently blank with no signal about what failed. Surface the
error in the pane and recover on the next compile. Also drop the
redundant canvasContext arg (pdf.js v6 renders via the canvas itself).

src/components/PdfPreview.tsx

@@ -1,4 +1,4 @@
-import { useEffect, useRef } from "react";
+import { useEffect, useRef, useState } from "react";
 import * as pdfjs from "pdfjs-dist";
 import workerUrl from "pdfjs-dist/build/pdf.worker.min.mjs?url";
 
@@ -6,6 +6,7 @@ pdfjs.GlobalWorkerOptions.workerSrc = workerUrl;
 
 export function PdfPreview({ data }: { data: Uint8Array }) {
   const ref = useRef<HTMLDivElement>(null);
+  const [error, setError] = useState<string | null>(null);
 
   useEffect(() => {
     const container = ref.current;
@@ -13,6 +14,7 @@ export function PdfPreview({ data }: { data: Uint8Array }) {
     let cancelled = false;
 
     (async () => {
+      try {
         const doc = await pdfjs.getDocument({ data: data.slice() }).promise;
         if (cancelled) return;
         const width = container.clientWidth;
@@ -30,12 +32,17 @@ export function PdfPreview({ data }: { data: Uint8Array }) {
           canvas.width = viewport.width;
           canvas.height = viewport.height;
           canvas.style.width = "100%";
-        const context = canvas.getContext("2d")!;
+          await page.render({ canvas, viewport }).promise;
-        await page.render({ canvas, canvasContext: context, viewport }).promise;
           if (cancelled) return;
           canvases.push(canvas);
         }
         container.replaceChildren(...canvases);
+        setError(null);
+      } catch (e) {
+        if (cancelled) return;
+        container.replaceChildren();
+        setError(String(e));
+      }
     })();
 
     return () => {
@@ -43,5 +50,10 @@ export function PdfPreview({ data }: { data: Uint8Array }) {
     };
   }, [data]);
 
-  return <div className="pdf-pages" ref={ref} />;
+  return (
+    <>
+      {error && <pre className="dock-error">{error}</pre>}
+      <div className="pdf-pages" ref={ref} />
+    </>
+  );
 }