refactor(pdf-viewer-tests): extract shared fake, add loadDocument error path, fix assertions

- Extract makeFakePdfjsLib / makeFakeLibLoader to testHelpers.ts — single
  source of truth used by both PdfViewer.svelte.test.ts and
  usePdfRenderer.svelte.test.ts; removes the diverging-fidelity DRY violation
  flagged by @felixbrandt and @saraholt in the PR review
- Add 'loadDocument sets error and loading=false when getDocument().promise
  rejects' test to usePdfRenderer.svelte.test.ts — closes the error-path gap
  flagged by @felixbrandt and @saraholt
- Replace toBeInTheDocument() with toBeVisible() in the three absorbed
  spec-file tests — uniform assertion style across the loaded-state describe
  block, as flagged by @felixbrandt

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

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

@@ -1,24 +1,6 @@
 import { describe, it, expect, vi } from 'vitest';
-import type { LibLoader } from './usePdfRenderer.svelte';
 import { createPdfRenderer } from './usePdfRenderer.svelte';
-
-function makeFakeLibLoader(): LibLoader {
-	return vi.fn().mockResolvedValue([
-		{
-			GlobalWorkerOptions: { workerSrc: '' },
-			getDocument: vi.fn().mockReturnValue({
-				promise: Promise.resolve({ numPages: 1, getPage: vi.fn() })
-			}),
-			TextLayer: class {
-				render() {
-					return Promise.resolve();
-				}
-				cancel() {}
-			}
-		} as unknown as typeof import('pdfjs-dist'),
-		{ default: '' }
-	] as const);
-}
+import { makeFakeLibLoader } from './testHelpers';
 
 // Note: init() and loadDocument() require pdfjsLib (browser module).
 // These tests cover pure state logic only — bounds clamping and zoom limits.
@@ -222,4 +204,24 @@ describe('createPdfRenderer', () => {
 		await r.init();
 		expect(fakeLoader).toHaveBeenCalledOnce();
 	});
+
+	it('loadDocument sets error and loading=false when getDocument().promise rejects', async () => {
+		const failingLib = {
+			GlobalWorkerOptions: { workerSrc: '' },
+			getDocument: vi.fn().mockReturnValue({
+				promise: Promise.reject(new Error('PDF not found'))
+			}),
+			TextLayer: class {
+				render() {
+					return Promise.resolve();
+				}
+				cancel() {}
+			}
+		} as unknown as typeof import('pdfjs-dist');
+		const r = createPdfRenderer(vi.fn().mockResolvedValue([failingLib, { default: '' }] as const));
+		await r.init();
+		await r.loadDocument('/bad/path');
+		expect(r.loading).toBe(false);
+		expect(r.error).toBe('PDF not found');
+	});
 });