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/pdf/render/render.worker.ts

@@ -0,0 +1,50 @@
+// Browser-only module worker: the one place react-pdf runs in the app. Not importable from Node tests.
+import { installStreamAsyncIterator } from "../../lib/streamPolyfill";
+import { initBrowserRendering } from "./browserInit";
+import { renderInvoicePdf } from "./invoice";
+import type { WorkerMessage, WorkerRequest } from "./jobs";
+
+interface WorkerScope {
+  onmessage: ((event: MessageEvent<WorkerRequest>) => void) | null;
+  postMessage(message: WorkerMessage, options?: { transfer: Transferable[] }): void;
+}
+const scope = self as unknown as WorkerScope;
+
+installStreamAsyncIterator();
+
+const post = (message: WorkerMessage, transfer: Transferable[] = []) => scope.postMessage(message, { transfer });
+
+const failure = (id: number | null, err: unknown): WorkerMessage => ({
+  type: "error",
+  id,
+  message: err instanceof Error ? err.message : String(err),
+  stack: err instanceof Error ? err.stack : undefined,
+});
+
+const started = initBrowserRendering();
+
+scope.onmessage = async (event) => {
+  const request = event.data;
+  if (request?.type !== "render") return;
+  const { job } = request;
+  try {
+    await started.catch(() => initBrowserRendering());
+    const t0 = performance.now();
+    const out = await renderInvoicePdf({ model: job.model, prefs: job.prefs });
+    const ms = Math.round(performance.now() - t0);
+    // Hand the buffer over instead of copying it; a view onto part of a larger buffer is copied first.
+    const whole = out.bytes.byteOffset === 0 && out.bytes.byteLength === out.bytes.buffer.byteLength;
+    const bytes = whole ? out.bytes : out.bytes.slice();
+    post(
+      { type: "result", result: { id: job.id, bytes, pages: out.pages, issues: out.issues, fingerprint: out.fingerprint, ms } },
+      [bytes.buffer],
+    );
+  } catch (err) {
+    post(failure(job.id, err));
+  }
+};
+
+started.then(
+  () => post({ type: "ready" }),
+  (err) => post(failure(null, err)),
+);