refactor(test): convert TextLayerMock to class syntax in PdfViewer spec

Prototype-style assignment was a vi.mock hoisting artifact from the old
version of the file. Rest of the codebase uses class syntax — aligning.

Addresses Felix Brandt round-4 suggestion on PR #536.

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

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

@@ -7,9 +7,12 @@ import PdfViewer from './PdfViewer.svelte';
 afterEach(cleanup);
 
 function makeFakePdfjsLib() {
-	function TextLayerMock() {}
-	TextLayerMock.prototype.render = () => Promise.resolve();
-	TextLayerMock.prototype.cancel = () => {};
+	class TextLayerMock {
+		render() {
+			return Promise.resolve();
+		}
+		cancel() {}
+	}
 
 	return {
 		GlobalWorkerOptions: { workerSrc: '' },