refactor(test): move PdfViewer import to top and annotate partial fake cast

- import PdfViewer left mid-file from vi.mock hoisting — no longer needed (Sara/Felix)
- adds one-line comment explaining as unknown as cast is an intentional partial fake (Felix)

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

frontend/src/lib/document/viewer/PdfViewer.svelte.spec.ts

@@ -2,6 +2,7 @@ import { vi, describe, it, expect, afterEach } from 'vitest';
 import { cleanup, render } from 'vitest-browser-svelte';
 import { page } from 'vitest/browser';
 import type { createPdfRenderer } from '$lib/document/viewer/usePdfRenderer.svelte';
+import PdfViewer from './PdfViewer.svelte';
 
 afterEach(cleanup);
 
@@ -23,6 +24,7 @@ function makeFakePdfjsLib() {
 			})
 		}),
 		TextLayer: TextLayerMock
+		// Partial fake: only the methods used by usePdfRenderer are implemented
 	} as unknown as typeof import('pdfjs-dist');
 }
 
@@ -31,8 +33,6 @@ function makeFakeLibLoader(): Parameters<typeof createPdfRenderer>[0] {
 	return vi.fn().mockResolvedValue([fakePdfjs, { default: '' }] as const);
 }
 
-import PdfViewer from './PdfViewer.svelte';
-
 describe('PdfViewer', () => {
 	it('shows previous and next page navigation buttons', async () => {
 		render(PdfViewer, { url: '/api/documents/test-id/file', libLoader: makeFakeLibLoader() });