Add searchable and flattened export, history re-export and diagnostics self-test

issueAndArchive keeps an issued invoice even when rendering or archiving
fails (Issued - not archived, with a Render & archive action). The export
menu offers a searchable PDF or a 300 DPI image-only PDF whose metadata is
the invoice number alone; flatten failures offer Retry or searchable export
and never silently lower the resolution. History re-exports the original
as issued by default and warns when a re-render's layout fingerprint
differs. The render client gains a FIFO assemble lane that previews cannot
supersede. A Diagnostics panel runs fixtures in the real worker and
compares against goldens generated from Node.

src/lib/browserFlatten.ts

@@ -0,0 +1,57 @@
+import { flattenPages, type CanvasLike, type FlattenResult, type PdfDocLike } from "../pdf/flatten";
+
+/** Lets the browser paint and handle input; falls back to a timer when rAF is paused (hidden window). */
+export const yieldToBrowser = (): Promise<void> =>
+  new Promise<void>((resolve) => {
+    let done = false;
+    const finish = () => {
+      if (!done) {
+        done = true;
+        resolve();
+      }
+    };
+    setTimeout(finish, 50);
+    requestAnimationFrame(() => setTimeout(finish, 0));
+  });
+
+/** JPEG bytes of a canvas via the browser's encoder. */
+export function encodeCanvasJpeg(canvas: CanvasLike, quality: number): Promise<Uint8Array> {
+  return new Promise((resolve, reject) => {
+    (canvas as unknown as HTMLCanvasElement).toBlob(
+      (blob) => {
+        if (!blob) return reject(new Error("the browser could not encode the page image (out of memory?)"));
+        blob.arrayBuffer().then((buf) => resolve(new Uint8Array(buf)), reject);
+      },
+      "image/jpeg",
+      quality,
+    );
+  });
+}
+
+export interface BrowserFlattenOptions {
+  signal?: AbortSignal;
+  onProgress?: (page: number, pages: number) => void;
+}
+
+/** Flattens PDF bytes at 300 DPI with pdf.js and a DOM canvas; opens and closes the document itself. */
+export async function flattenInBrowser(bytes: Uint8Array, opts: BrowserFlattenOptions = {}): Promise<FlattenResult> {
+  const { openPdf, closePdf } = await import("./pdfjs");
+  const doc = await openPdf(bytes);
+  try {
+    return await flattenPages({
+      pdf: doc as unknown as PdfDocLike,
+      createCanvas: (width, height) => {
+        const canvas = document.createElement("canvas");
+        canvas.width = width;
+        canvas.height = height;
+        return canvas as unknown as CanvasLike;
+      },
+      encodeJpeg: encodeCanvasJpeg,
+      yieldToUi: yieldToBrowser,
+      signal: opts.signal,
+      onProgress: opts.onProgress,
+    });
+  } finally {
+    await closePdf(doc).catch(() => {});
+  }
+}