Replace the HTML preview with a worker-rendered PDF preview

Rendering moves into a module Web Worker behind a latest-wins RenderClient
with a watchdog and a handshake-based main-thread fallback. The preview
shows the real PDF through pdf.js at 2x with zoom, page indicator, audit
issues popover and a focus modal. A4/Letter and margin controls are frozen
into each invoice at issue and can be saved as the default (migration M3).
react-pdf and pdf.js are lazy-loaded; the main chunk shrinks from 1.8 MB
to 0.5 MB. The HTML twin and its styles are removed.

src/lib/renderClient.ts

@@ -0,0 +1,12 @@
+import { createAutoClient, type RenderClient } from "../pdf/render/client";
+
+let client: RenderClient | null = null;
+let lastId = 0;
+
+/** The one render client (and so one worker) shared by the preview and export. */
+export function getRenderClient(): RenderClient {
+  client ??= createAutoClient();
+  return client;
+}
+
+export const nextRenderJobId = (): number => ++lastId;