fix(boot): a stylesheet lost on the way is asked for again

A lost stylesheet left the page unstyled until a reload, and every e2e step that finds elements by
testid still passed on it. Only axe noticed: the one-off target-size failure on eight scattered
controls. Losing the main stylesheet on purpose gives those same eight.

frontend/src/lib/stylesheetRetry.ts

@@ -0,0 +1,50 @@
+// A stylesheet can be lost on the way (a poor line, a loaded machine), and a browser never asks again
+// for one by itself: the page then stays unstyled until a reload, every control a bare browser button.
+// A module script waits for the document's stylesheets (measured: a sheet held 3 s held the module
+// too), so by the time this runs each link has either loaded or failed, and no event is left to wait
+// for. A lost one is asked for again in its own place, so the cascade's order stays as it was built.
+//
+// All of these sheets are this site's own: reading the rules of a loaded one is allowed, so a throw
+// says it failed (measured: Chromium, WebKit). Three tries, then the page stays as it is. A reload here
+// could loop, and the unstyled page still works.
+import { log } from "./log";
+
+const RETRY_AFTER_MS = [0, 1000, 3000];
+
+function holdsRules(link: HTMLLinkElement): boolean {
+  try {
+    return (link.sheet?.cssRules.length ?? 0) > 0;
+  } catch {
+    return false;
+  }
+}
+
+function askAgain(link: HTMLLinkElement, attempt: number): void {
+  const wait = RETRY_AFTER_MS[attempt];
+  if (wait === undefined) {
+    log.error("app.stylesheet_gave_up", "a stylesheet could not be loaded", { href: link.href });
+    return;
+  }
+  window.setTimeout(() => {
+    const next = link.cloneNode(false) as HTMLLinkElement;
+    next.addEventListener("load", () =>
+      log.info("app.stylesheet_recovered", "a lost stylesheet loaded on a new request", {
+        href: next.href,
+        attempt: attempt + 1,
+      }),
+    );
+    next.addEventListener("error", () => askAgain(next, attempt + 1));
+    link.replaceWith(next);
+  }, wait);
+}
+
+/** Run once, from a page's entry module. */
+export function retryLostStylesheets(): void {
+  for (const link of document.querySelectorAll<HTMLLinkElement>('link[rel="stylesheet"]')) {
+    if (holdsRules(link)) continue;
+    log.warn("app.stylesheet_lost", "a stylesheet was lost on the way; asking again", {
+      href: link.href,
+    });
+    askAgain(link, 0);
+  }
+}