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/__tests__/app-version-store.test.ts

@@ -0,0 +1,46 @@
+import { describe, it, expect, beforeEach } from "vitest";
+import {
+  reportServerVersion,
+  subscribeNewVersion,
+  getNewVersion,
+  BUILD_VERSION,
+  __resetVersionStateForTest,
+} from "../admin/utils/appVersion";
+
+/**
+ * Client store backing the "new version available" banner. The server stamps
+ * X-App-Version on every response; apiFetch feeds it here. A version that
+ * differs from the one baked into THIS bundle (BUILD_VERSION) means a deploy
+ * happened → latch it and notify subscribers (the banner).
+ */
+
+describe("appVersion store", () => {
+  beforeEach(() => __resetVersionStateForTest());
+
+  it("ignores the same version, empty, or null", () => {
+    reportServerVersion(BUILD_VERSION);
+    reportServerVersion("");
+    reportServerVersion(null);
+    reportServerVersion(undefined);
+    expect(getNewVersion()).toBeNull();
+  });
+
+  it("latches + notifies once when the server reports a different version", () => {
+    let notified = 0;
+    const unsub = subscribeNewVersion(() => {
+      notified++;
+    });
+
+    reportServerVersion(`${BUILD_VERSION}-next`);
+    expect(getNewVersion()).toBe(`${BUILD_VERSION}-next`);
+    expect(notified).toBe(1);
+
+    // Idempotent: once a new version is known, further reports don't re-notify
+    // or overwrite (the banner is already showing).
+    reportServerVersion("99.0.0");
+    expect(getNewVersion()).toBe(`${BUILD_VERSION}-next`);
+    expect(notified).toBe(1);
+
+    unsub();
+  });
+});

src/__tests__/version-header.test.ts

@@ -0,0 +1,24 @@
+import { describe, it, expect } from "vitest";
+import Fastify from "fastify";
+import { registerVersionHeader, readAppVersion } from "../middleware/version";
+
+/**
+ * The server stamps every response with X-App-Version so the SPA can detect a
+ * deploy (the client compares it to the version baked into its bundle).
+ */
+
+describe("registerVersionHeader", () => {
+  it("adds X-App-Version to responses", async () => {
+    const app = Fastify({ logger: false });
+    registerVersionHeader(app, "9.9.9");
+    app.get("/ping", async () => ({ ok: true }));
+
+    const res = await app.inject({ method: "GET", url: "/ping" });
+    expect(res.headers["x-app-version"]).toBe("9.9.9");
+    await app.close();
+  });
+
+  it("defaults to the package.json version (semver string)", () => {
+    expect(readAppVersion()).toMatch(/^\d+\.\d+\.\d+/);
+  });
+});