fix(app): reload onto a new build instead of crashing on a missing chunk

A tab left open across a release asked for a lazy chunk the server no
longer has (prod: DownloadDialog 404 an hour after 0.89) and fell to the
crash screen. `vite:preloadError` now reloads the tab once a minute at
most; a second failure is let through to the error boundary, which now
wraps every entry — the public /watch, /privacy and /terms pages crashed
to a blank page.

frontend/src/lib/staleBuild.dom.test.ts

@@ -0,0 +1,44 @@
+// @vitest-environment jsdom
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+
+const fire = (): Event => {
+  const e = new Event("vite:preloadError", { cancelable: true });
+  window.dispatchEvent(e);
+  return e;
+};
+
+let uninstall = () => {};
+
+describe("a lazy chunk the server no longer has", () => {
+  afterEach(() => uninstall());
+  beforeEach(() => {
+    vi.resetModules();
+    sessionStorage.clear();
+  });
+
+  it("reloads the tab once, and says so to the error boundary", async () => {
+    const { installStaleBuildReload, reloadingForNewBuild } = await import("./staleBuild");
+    const reload = vi.fn();
+    uninstall = installStaleBuildReload(reload);
+
+    const e = fire();
+
+    expect(e.defaultPrevented).toBe(true);
+    expect(reload).toHaveBeenCalledTimes(1);
+    expect(reloadingForNewBuild()).toBe(true);
+    // The rest of the page's failed imports while the reload is under way: swallowed, no second reload.
+    expect(fire().defaultPrevented).toBe(true);
+    expect(reload).toHaveBeenCalledTimes(1);
+  });
+
+  it("lets the error through when the reload a moment ago did not help", async () => {
+    sessionStorage.setItem("sf.staleBuildReload", String(Date.now() - 5_000));
+    const { installStaleBuildReload, reloadingForNewBuild } = await import("./staleBuild");
+    const reload = vi.fn();
+    uninstall = installStaleBuildReload(reload);
+
+    expect(fire().defaultPrevented).toBe(false);
+    expect(reload).not.toHaveBeenCalled();
+    expect(reloadingForNewBuild()).toBe(false);
+  });
+});