feat(app): detect new deploy and prompt refresh (+ lazy-chunk recovery)

When a new version is deployed, open tabs no longer silently run stale code
until a manual F5.

- Server stamps every response with X-App-Version (src/middleware/version.ts,
  onSend hook in server.ts).
- Client bakes its build version via Vite define (__APP_VERSION__); apiFetch
  compares the header to it (no polling — piggybacks existing traffic) and
  latches a mismatch in a small store (appVersion.ts).
- UpdateBanner: a persistent bottom Snackbar "Je k dispozici nová verze
  aplikace. — Obnovit" → location.reload(). User-initiated, so an in-progress
  form is never interrupted.
- lazyWithReload: every React.lazy page reloads once if its hashed chunk 404s
  after a deploy (sessionStorage-guarded against loops), fixing the "old chunk
  gone" error screen.

Approach chosen after researching current SPA practice (version header + banner
beats polling / a service worker for a non-PWA admin app).

Tests: +4 (server header, client store latch/notify). Full suite 669 pass,
tsc -b clean, lint 0. Verified live via Playwright (header emitted; banner
appears on a simulated mismatch). Detection applies to deploys from the NEXT
release onward (current clients lack the detection code).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/admin/components/UpdateBanner.tsx

@@ -0,0 +1,35 @@
+import { useSyncExternalStore } from "react";
+import Snackbar from "@mui/material/Snackbar";
+import Button from "@mui/material/Button";
+import { subscribeNewVersion, getNewVersion } from "../utils/appVersion";
+
+/**
+ * Shows a persistent banner when the server reports a newer build than the one
+ * this tab is running (detected via the X-App-Version header in apiFetch). The
+ * reload is user-initiated — we never force it, so an in-progress form is safe.
+ */
+export default function UpdateBanner() {
+  const newVersion = useSyncExternalStore(
+    subscribeNewVersion,
+    getNewVersion,
+    () => null,
+  );
+
+  return (
+    <Snackbar
+      open={Boolean(newVersion)}
+      anchorOrigin={{ vertical: "bottom", horizontal: "center" }}
+      message="Je k dispozici nová verze aplikace."
+      action={
+        <Button
+          color="primary"
+          size="small"
+          variant="contained"
+          onClick={() => window.location.reload()}
+        >
+          Obnovit
+        </Button>
+      }
+    />
+  );
+}