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/server.ts

@@ -206,12 +206,30 @@ async function start() {
       root: path.join(__dirname, "..", "dist-client"),
       prefix: "/",
       wildcard: false,
+      // Deploy-skew caching contract (per the Vite deploy guide):
+      // - /assets/* names are content-hashed → safe to cache forever
+      //   (a changed file always gets a NEW name, so "immutable" is correct).
+      // - index.html (and any non-hashed file) must always be revalidated,
+      //   otherwise a cached copy keeps referencing deleted old chunks.
+      setHeaders: (res, filePath) => {
+        if (filePath.includes(`${path.sep}assets${path.sep}`)) {
+          res.setHeader("Cache-Control", "public, max-age=31536000, immutable");
+        } else {
+          res.setHeader("Cache-Control", "no-cache");
+        }
+      },
     });
 
     app.setNotFoundHandler((request, reply) => {
       if (request.url.startsWith("/api/")) {
         return reply.status(404).send({ success: false, error: "Not found" });
       }
+      // A missing hashed asset (old chunk after a deploy) must 404, NOT get
+      // the SPA index.html fallback — HTML-as-JS masks the failure and breaks
+      // the client's vite:preloadError reload recovery.
+      if (request.url.startsWith("/assets/")) {
+        return reply.status(404).send();
+      }
       return reply.sendFile("index.html");
     });
   }