fix: incremental PDF page rendering for exam setup backdrop

Rendering a 36-page AQA exam PDF sequentially created 36 large canvas
elements, causing memory pressure and keeping pdfStatus='loading' for
60–120s in headless Chrome. Two changes:

1. pdfLoader.ts: reuse a single canvas (reduces peak memory from ~120MB
   to ~4MB) and fire onPageReady callback after each page so callers can
   stream pages to the canvas as they render.

2. ExamTemplateSetupPage.tsx: use the callback to add each PDF page
   shape to the tldraw canvas the moment it renders, making the first
   page visible within a few seconds rather than after all pages load.

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

src/pages/exam/setup/pdfLoader.ts

@@ -12,20 +12,27 @@ export interface PdfPageImage {
   height: number
 }
 
-export async function loadPdfPageImages(pdfBytes: ArrayBuffer, targetWidth = PAGE_WIDTH): Promise<PdfPageImage[]> {
+export async function loadPdfPageImages(
+  pdfBytes: ArrayBuffer,
+  targetWidth = PAGE_WIDTH,
+  onPageReady?: (pages: PdfPageImage[]) => void,
+): Promise<PdfPageImage[]> {
   const pdf = await pdfjsLib.getDocument({ data: new Uint8Array(pdfBytes) }).promise
   const pages: PdfPageImage[] = []
+  // Reuse a single canvas across all pages to avoid allocating ~120 MB of canvas memory
+  // for a typical 36-page exam paper.
+  const canvas = document.createElement("canvas")
 
   for (let pageNumber = 1; pageNumber <= pdf.numPages; pageNumber += 1) {
     const page = await pdf.getPage(pageNumber)
     const baseViewport = page.getViewport({ scale: 1 })
     const scale = targetWidth / baseViewport.width
     const viewport = page.getViewport({ scale })
-    const canvas = document.createElement("canvas")
     canvas.width = Math.ceil(viewport.width)
     canvas.height = Math.ceil(viewport.height)
     const context = canvas.getContext("2d")
     if (!context) throw new Error("Unable to create PDF render canvas")
+    context.clearRect(0, 0, canvas.width, canvas.height)
     await page.render({ canvasContext: context, viewport }).promise
     pages.push({
       pageNumber,
@@ -33,6 +40,7 @@ export async function loadPdfPageImages(pdfBytes: ArrayBuffer, targetWidth = PAG
       width: canvas.width,
       height: canvas.height,
     })
+    onPageReady?.([...pages])
   }
 
   return pages