fix(document): pass wasmUrl to pdf.js getDocument so wasm decoders load

getDocument was called with a bare src string, so pdf.js 5.x had no
`wasmUrl` and could not initialise the JBIG2/CCITTFax wasm decoder —
CCITT (G4 fax) scans painted a blank canvas. Pass
{ url, wasmUrl: '/pdfjs-wasm/' }; the directory URL (trailing slash
required) is the single source of truth next to the worker config.

Refs #708

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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

@@ -205,6 +205,32 @@ describe('createPdfRenderer', () => {
 		expect(fakeLoader).toHaveBeenCalledOnce();
 	});
 
+	it('passes a non-null wasmUrl directory (ending in /) to getDocument, not a bare src string', async () => {
+		const getDocument = vi.fn().mockReturnValue({
+			promise: Promise.resolve({ numPages: 1, getPage: vi.fn() })
+		});
+		const lib = {
+			GlobalWorkerOptions: { workerSrc: '' },
+			getDocument,
+			TextLayer: class {
+				render() {
+					return Promise.resolve();
+				}
+				cancel() {}
+			}
+		} as unknown as typeof import('pdfjs-dist');
+		const r = createPdfRenderer(vi.fn().mockResolvedValue([lib, { default: '' }] as const));
+		await r.init();
+		await r.loadDocument('/api/documents/abc/file');
+
+		expect(getDocument).toHaveBeenCalledTimes(1);
+		const arg = getDocument.mock.calls[0][0] as { url?: string; wasmUrl?: string };
+		expect(arg.url).toBe('/api/documents/abc/file');
+		expect(typeof arg.wasmUrl).toBe('string');
+		expect(arg.wasmUrl).not.toBe('');
+		expect(arg.wasmUrl?.endsWith('/')).toBe(true);
+	});
+
 	it('loadDocument sets error and loading=false when getDocument().promise rejects', async () => {
 		const failingLib = {
 			GlobalWorkerOptions: { workerSrc: '' },