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/views/AppSettings.tsx

@@ -4,6 +4,7 @@ import {
   Column,
   Grid,
   InlineNotification,
+  Link,
   NumberInput,
   RadioButton,
   RadioButtonGroup,
@@ -32,6 +33,7 @@ import {
 } from "../lib/types";
 import { emailError, ifscError, panError, phoneError, vendorGstinError } from "../lib/validators";
 import { ImagePicker } from "../components/ImagePicker";
+import DiagnosticsPanel from "../components/DiagnosticsPanel";
 
 const blankBank = (): BankAccount => ({
   id: null,
@@ -52,6 +54,7 @@ export default function AppSettings({
   onSaved: () => void;
 }) {
   const [form, setForm] = useState<Settings>({ ...settings });
+  const [diagnosticsOpen, setDiagnosticsOpen] = useState(false);
   const [banks, setBanks] = useState<BankAccount[]>([]);
   const [editing, setEditing] = useState<BankAccount | null>(null);
   const [busy, setBusy] = useState(false);
@@ -474,6 +477,20 @@ export default function AppSettings({
       <Button renderIcon={Save} onClick={saveAll} disabled={busy}>
         {busy ? "Saving…" : "Save settings"}
       </Button>
+
+      <p className="voiced-muted" style={{ marginTop: "3rem", fontSize: "0.75rem" }}>
+        <Link
+          href="#"
+          size="sm"
+          onClick={(e: { preventDefault: () => void }) => {
+            e.preventDefault();
+            setDiagnosticsOpen(true);
+          }}
+        >
+          Diagnostics
+        </Link>
+      </p>
+      <DiagnosticsPanel open={diagnosticsOpen} onClose={() => setDiagnosticsOpen(false)} />
     </div>
   );
 }