test(pdf-renderer): assert init() re-throws when libLoader rejects

.catch(()=>{}) swallowed the rejection, so the test passed vacuously even
if a future refactor silently caught the error. rejects.toThrow() proves
the propagation contract holds before asserting pdfjsReady stays false.

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

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

@@ -190,7 +190,7 @@ describe('createPdfRenderer', () => {
 	it('leaves pdfjsReady false when libLoader rejects', async () => {
 		const failingLoader = vi.fn().mockRejectedValue(new Error('load failed'));
 		const r = createPdfRenderer(failingLoader);
-		await r.init().catch(() => {});
+		await expect(r.init()).rejects.toThrow('load failed');
 		expect(r.pdfjsReady).toBe(false);
 	});
 });