test(pdf-renderer): guard init() against repeated calls — libLoader must fire once

Adds idempotency test: calling init() twice must invoke libLoader only once.
Adds `if (pdfjsReady) return;` guard to satisfy the contract.

Addresses Felix Brandt round-4 suggestion on PR #536.

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

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

@@ -23,6 +23,7 @@ export function createPdfRenderer(libLoader: LibLoader = defaultLibLoader) {
 	let pdfjsLib: typeof import('pdfjs-dist') | null = null;
 
 	async function init(): Promise<void> {
+		if (pdfjsReady) return;
 		const [lib, { default: workerUrl }] = await libLoader();
 		lib.GlobalWorkerOptions.workerSrc = workerUrl;
 		pdfjsLib = lib;