fix(web): blank-screen hardening — stale chunk recovery, error boundary, no-cache index.html

- main.tsx: on vite:preloadError (stale index.html referencing removed
  hashed chunks after a redeploy) reload the page immediately.
- New ErrorBoundary around the whole router: render errors show a Persian
  fallback with «تلاش مجدد» instead of wiping the DOM.
- nginx: index.html served with Cache-Control no-cache,no-store,
  must-revalidate (verified live); /assets/* stays public, immutable.

frontend/nginx.conf

@@ -34,7 +34,14 @@ server {
         try_files $uri $uri/ /index.html;
     }
 
-    # Static assets cache
+    # index.html itself must never be cached: after a redeploy the hashed
+    # /assets/* names change, and a stale cached HTML would 404 on the old
+    # chunks (blank page). Content-hashed assets below stay immutable.
+    location = /index.html {
+        add_header Cache-Control "no-cache, no-store, must-revalidate";
+    }
+
+    # Static assets cache — hashed filenames are content-addressed
     location /assets/ {
         expires 30d;
         add_header Cache-Control "public, immutable";