fix(document): make the PDF error state accessible (alert + larger link)

The error block was a colour-only, visually-small dead end. Add
role="alert" so screen readers announce the failure, bump the message to
text-base and the recovery download link to text-sm with a py-2 tap
target — the only escape hatch, sized for the archive's older readers.

Addresses re-review: Leonie (a11y of the error state).

Refs #708

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

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

@@ -44,6 +44,8 @@ describe('PdfViewer — render failure', () => {
 
 		await expect.element(page.getByText(m.doc_render_failed())).toBeVisible();
 		await expect.element(page.getByRole('link', { name: m.doc_download_link() })).toBeVisible();
+		// Announced to assistive tech, not a silent visual-only failure.
+		await expect.element(page.getByRole('alert')).toBeVisible();
 	});
 
 	it('does not show the failure message when the page renders successfully', async () => {