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/pdf.tsx

@@ -1,4 +1,3 @@
-import { save } from "@tauri-apps/plugin-dialog";
 import pkg from "../../package.json";
 import { parsePrefs, type RenderPrefsV1 } from "../pdf/model/prefs";
 import { buildRenderModel, type RenderModel } from "../pdf/model/build";
@@ -57,6 +56,11 @@ export async function renderInvoicePdf(
   prefs: RenderPrefsV1 = prefsForInvoice(invoice, settings),
 ): Promise<RenderResult> {
   const model = await buildModelForInvoice(invoice, settings, bank);
+  return renderModelWithRetry(model, prefs);
+}
+
+/** Renders a ready model in the shared client. */
+export async function renderModelWithRetry(model: RenderModel, prefs: RenderPrefsV1): Promise<RenderResult> {
   // The live preview shares the client; if it takes the worker first this render is superseded, so go again.
   for (let attempt = 0; ; attempt++) {
     try {
@@ -66,25 +70,3 @@ export async function renderInvoicePdf(
     }
   }
 }
-
-/** Asks for a save location and writes the bytes there; null when the dialog is cancelled. */
-export async function savePdfBytes(bytes: Uint8Array, fileName: string): Promise<string | null> {
-  const suggested = fileName.replace(/[^A-Za-z0-9._-]+/g, "-") || "invoice";
-  const path = await save({
-    defaultPath: `${suggested}.pdf`,
-    filters: [{ name: "PDF document", extensions: ["pdf"] }],
-  });
-  if (!path) return null;
-  await api.writeExportFile(path, bytes);
-  return path;
-}
-
-export async function exportInvoicePdf(
-  invoice: Invoice,
-  settings: Settings,
-  bank: BankAccount | null,
-  prefs?: RenderPrefsV1,
-): Promise<string | null> {
-  const { bytes } = await renderInvoicePdf(invoice, settings, bank, prefs);
-  return savePdfBytes(bytes, invoice.number);
-}