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/exportMode.ts

@@ -0,0 +1,27 @@
+import type { ExportMode } from "./filename";
+
+const KEY = "voiced.exportMode";
+
+/** The last export mode this viewer used; searchable until one is chosen. A per-viewer convenience only. */
+export function loadExportMode(): ExportMode {
+  try {
+    const v = localStorage.getItem(KEY);
+    if (v === "searchable" || v === "flattened") return v;
+  } catch {
+    // Storage can be blocked or empty; the default is fine.
+  }
+  return "searchable";
+}
+
+export function saveExportMode(mode: ExportMode): void {
+  try {
+    localStorage.setItem(KEY, mode);
+  } catch {
+    // Not remembering is harmless.
+  }
+}
+
+export const EXPORT_MODE_LABEL: Record<ExportMode, string> = {
+  searchable: "Searchable PDF (copy-pasteable text)",
+  flattened: "Flattened PDF (image only)",
+};