test(pdf-renderer): inject libLoader into createPdfRenderer to eliminate vi.mock factories

Adds an optional LibLoader parameter (defaults to the real pdfjs-dist dynamic
imports) and a failing test that verified the loader is called during init().
This is the first step toward removing ManualMockedModule registrations that
race with vitest-browser-playwright's birpc teardown (#535).

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

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

@@ -1,4 +1,4 @@
-import { describe, it, expect } from 'vitest';
+import { describe, it, expect, vi } from 'vitest';
 import { createPdfRenderer } from './usePdfRenderer.svelte';
 
 // Note: init() and loadDocument() require pdfjsLib (browser module).
@@ -66,4 +66,17 @@ describe('createPdfRenderer', () => {
 		expect(r.error).toBeNull();
 		expect(r.loading).toBe(false);
 	});
+
+	it('calls injected libLoader during init and sets pdfjsReady', 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();
+		expect(fakeLoader).toHaveBeenCalledOnce();
+		expect(r.pdfjsReady).toBe(true);
+	});
 });