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.test.ts

@@ -193,4 +193,17 @@ describe('createPdfRenderer', () => {
 		await expect(r.init()).rejects.toThrow('load failed');
 		expect(r.pdfjsReady).toBe(false);
 	});
+
+	it('init() is idempotent — libLoader called only once on repeated calls', async () => {
+		const fakePdfjs = {
+			GlobalWorkerOptions: { workerSrc: '' },
+			getDocument: vi.fn(),
+			TextLayer: class {}
+		} as unknown as typeof import('pdfjs-dist');
+		const fakeLoader = vi.fn().mockResolvedValue([fakePdfjs, { default: '' }] as const);
+		const r = createPdfRenderer(fakeLoader);
+		await r.init();
+		await r.init();
+		expect(fakeLoader).toHaveBeenCalledOnce();
+	});
 });