refactor(pdf-viewer): export LibLoader type and update callers

Exporting LibLoader gives the type a stable, named identity.
PdfViewer.svelte and PdfViewer.svelte.spec.ts now import it directly
instead of using Parameters<typeof createPdfRenderer>[0].

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

frontend/src/lib/document/viewer/usePdfRenderer.svelte.ts

@@ -1,6 +1,6 @@
 import type { PDFDocumentProxy, RenderTask } from 'pdfjs-dist';
 
-type LibLoader = () => Promise<readonly [typeof import('pdfjs-dist'), { default: string }]>;
+export type LibLoader = () => Promise<readonly [typeof import('pdfjs-dist'), { default: string }]>;
 
 const defaultLibLoader: LibLoader = () =>
 	Promise.all([import('pdfjs-dist'), import('pdfjs-dist/build/pdf.worker.min.mjs?url')]);