fix(spa): self-heal stale clients after deploys (vite:preloadError + cache headers)

After a release the old hashed chunks are deleted, so an open tab from
the previous build failed its next lazy-page import until a manual
refresh. Three-part fix per the Vite deploy guide:

- vite:preloadError listener reloads the page once (10s sessionStorage
  guard against loops) so stale tabs heal themselves
- static cache headers: /assets/* immutable for a year (content-hashed
  names), index.html no-cache so new visits always see fresh chunk refs
- missing /assets/* now 404 instead of getting the SPA index.html
  fallback (HTML-as-JS masked the failure)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/main.tsx

@@ -4,6 +4,22 @@ import { BrowserRouter } from "react-router-dom";
 import App from "./App";
 import { ThemeProvider } from "./context/ThemeContext";
 
+// Deploy-skew recovery (official Vite mechanism): after a release the old
+// hashed chunks are deleted, so a tab still running the previous build fails
+// its next lazy-page import. Vite emits `vite:preloadError` for exactly this —
+// swallow the error and reload once, which loads the fresh index.html and new
+// chunk names. The timestamp guard prevents a reload loop when a chunk is
+// genuinely unloadable (e.g. offline): a second failure within 10 s falls
+// through to the normal error path.
+window.addEventListener("vite:preloadError", (event) => {
+  const KEY = "vite-preload-reloaded-at";
+  const last = Number(sessionStorage.getItem(KEY) || 0);
+  if (Date.now() - last < 10_000) return;
+  sessionStorage.setItem(KEY, String(Date.now()));
+  event.preventDefault();
+  window.location.reload();
+});
+
 ReactDOM.createRoot(document.getElementById("root")!).render(
   <React.StrictMode>
     <BrowserRouter