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/styles/carbon.scss

@@ -353,3 +353,46 @@ body {
   background-color: var(--cds-support-warning);
   color: #161616;
 }
+
+/* ---------- export feedback ---------- */
+.voiced-notice-actions {
+  display: flex;
+  flex-wrap: wrap;
+  gap: 0.5rem;
+  margin-top: 0.5rem;
+}
+
+.voiced-export-split {
+  display: inline-flex;
+  align-items: stretch;
+}
+
+.voiced-diagnostics__check {
+  padding: 0.5rem 0;
+  border-bottom: 1px solid var(--cds-border-subtle-01);
+}
+
+.voiced-diagnostics__head {
+  display: flex;
+  align-items: center;
+  gap: 0.5rem;
+}
+
+.voiced-diagnostics__label {
+  flex: 1 1 auto;
+  min-width: 0;
+}
+
+.voiced-diagnostics__ms {
+  color: var(--cds-text-secondary);
+  font-size: 0.75rem;
+  white-space: nowrap;
+}
+
+.voiced-diagnostics__detail {
+  margin: 0.25rem 0 0;
+  color: var(--cds-text-secondary);
+  font-size: 0.75rem;
+  white-space: pre-wrap;
+  overflow-wrap: anywhere;
+}