fix(frontend): chunk PDFs are single-page — always render page 1

Chunk PDFs like 'NEC4 ACC_page_5.pdf' contain only 1 page (the extracted
page). Passing the original page number (e.g. 5) to <Page> caused
'Invalid page request' because the file only has 1 page.

- Always render pageNumber={1} for <Page> component
- Display originalPage in UI title to show source page number
- Disable prev/next navigation since chunk PDFs are single-page

frontend/src/pages/PdfViewerPage.tsx

@@ -13,11 +13,11 @@ const DEFAULT_ZOOM_INDEX = 2
 export const PdfViewerPage: React.FC = () => {
   const [searchParams] = useSearchParams()
   const pdfUrl = searchParams.get('url') ?? ''
-  const initialPage = parseInt(searchParams.get('page') ?? '1', 10)
+  const originalPage = parseInt(searchParams.get('page') ?? '1', 10)
   const title = searchParams.get('title') ?? 'PDF Viewer'
 
   const [numPages, setNumPages] = useState<number>(0)
-  const [pageNumber, setPageNumber] = useState<number>(initialPage)
+  const [pageNumber] = useState<number>(1)
   const [zoomIndex, setZoomIndex] = useState<number>(DEFAULT_ZOOM_INDEX)
   const [loadError, setLoadError] = useState<string | null>(null)
   const [pageError, setPageError] = useState<string | null>(null)
@@ -36,8 +36,6 @@ export const PdfViewerPage: React.FC = () => {
     setPageError(error.message)
   }, [])
 
-  const goToPrevPage = () => setPageNumber((prev) => Math.max(1, prev - 1))
-  const goToNextPage = () => setPageNumber((prev) => Math.min(numPages, prev + 1))
   const zoomIn = () => setZoomIndex((prev) => Math.min(ZOOM_LEVELS.length - 1, prev + 1))
   const zoomOut = () => setZoomIndex((prev) => Math.max(0, prev - 1))
 
@@ -61,19 +59,17 @@ export const PdfViewerPage: React.FC = () => {
         </Link>
         <div className="flex items-center space-x-2">
           <button
-            onClick={goToPrevPage}
-            disabled={pageNumber <= 1}
-            className="p-1 rounded hover:bg-gray-100 disabled:opacity-30 disabled:cursor-not-allowed"
+            disabled
+            className="p-1 rounded disabled:opacity-30 disabled:cursor-not-allowed"
           >
             <ChevronLeft className="w-4 h-4" />
           </button>
           <span className="text-sm text-gray-700 min-w-[120px] text-center">
-            {title} — Page {pageNumber}{numPages > 0 ? ` of ${numPages}` : ''}
+            {title} — Page {originalPage}
           </span>
           <button
-            onClick={goToNextPage}
-            disabled={pageNumber >= numPages}
-            className="p-1 rounded hover:bg-gray-100 disabled:opacity-30 disabled:cursor-not-allowed"
+            disabled
+            className="p-1 rounded disabled:opacity-30 disabled:cursor-not-allowed"
           >
             <ChevronRight className="w-4 h-4" />
           </button>