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/streamPolyfill.ts

@@ -0,0 +1,25 @@
+/**
+ * WebKitGTK (2.50) lacks ReadableStream.prototype[Symbol.asyncIterator], which pdf.js uses in
+ * getTextContent. Installs a minimal version when it is missing; a no-op elsewhere.
+ */
+export function installStreamAsyncIterator(): void {
+  const proto = (globalThis as { ReadableStream?: { prototype: object } }).ReadableStream?.prototype as
+    | Record<symbol, unknown>
+    | undefined;
+  if (!proto || Symbol.asyncIterator in proto) return;
+  proto[Symbol.asyncIterator] = function (this: ReadableStream<unknown>) {
+    const reader = this.getReader();
+    const iterator = {
+      next: () => reader.read(),
+      return: async () => {
+        await reader.cancel().catch(() => {});
+        reader.releaseLock();
+        return { done: true as const, value: undefined };
+      },
+      [Symbol.asyncIterator]() {
+        return iterator;
+      },
+    };
+    return iterator;
+  };
+}