fix(pdf-viewer): replace vi.mock(pdfjs-dist) with injected libLoader prop

Removes both vi.mock('pdfjs-dist', factory) and
vi.mock('pdfjs-dist/build/pdf.worker.min.mjs?url', factory) from
PdfViewer.svelte.spec.ts — the ManualMockedModule registrations that were
racing with vitest-browser-playwright's birpc teardown channel.

PdfViewer.svelte now accepts an optional libLoader prop (typed as
Parameters<typeof createPdfRenderer>[0]) that is passed untracked to
createPdfRenderer(). Tests supply a vi.fn() fake loader directly as a prop;
production code uses the default loader that imports the real pdfjs-dist.
The birpc route handler for pdfjs-dist is never registered, so no teardown
race is possible. Fixes #535.

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

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

@@ -1,5 +1,5 @@
 <script lang="ts">
-import { onMount, setContext } from 'svelte';
+import { onMount, setContext, untrack } from 'svelte';
 import { createPdfRenderer } from '$lib/document/viewer/usePdfRenderer.svelte';
 import PdfControls from './PdfControls.svelte';
 import AnnotationLayer from '$lib/document/annotation/AnnotationLayer.svelte';
@@ -21,7 +21,8 @@ let {
 	onDeleteAnnotationRequest,
 	documentFileHash,
 	annotationsDimmed = false,
-	flashAnnotationId = null
+	flashAnnotationId = null,
+	libLoader = undefined
 }: {
 	url: string;
 	documentId?: string;
@@ -35,9 +36,10 @@ let {
 	documentFileHash?: string | null;
 	annotationsDimmed?: boolean;
 	flashAnnotationId?: string | null;
+	libLoader?: Parameters<typeof createPdfRenderer>[0];
 } = $props();
 
-const renderer = createPdfRenderer();
+const renderer = untrack(() => createPdfRenderer(libLoader));
 
 // Canvas and text layer container refs — bound via bind:this
 let canvasEl = $state<HTMLCanvasElement | null>(null);