Stop caching index.html and speed up socket reconnect

Phones offer no easy hard-refresh, so a cached index.html kept serving an old
bundle after a deploy (the live feed showed "reconnecting"). index.html and the
SPA catch-all now send Cache-Control: no-store, while hashed /assets stay
immutable. Also retry the first socket reconnect after 1s (was 2s) and surface
the close code in the header so a failure is actionable on a phone.

frontend/nginx.conf

@@ -4,7 +4,22 @@ server {
     root /usr/share/nginx/html;
     index index.html;
 
+    # Hashed Vite assets are immutable; cache them hard.
+    location /assets/ {
+        add_header Cache-Control "public, max-age=31536000, immutable";
+        try_files $uri =404;
+    }
+
+    # The HTML entry point must never be cached, or a phone (which offers no
+    # easy hard-refresh) keeps loading an old bundle after a deploy. The
+    # catch-all serves index.html for client-side routes, so it gets the same
+    # no-store header.
+    location = /index.html {
+        add_header Cache-Control "no-store";
+    }
+
     location / {
+        add_header Cache-Control "no-store";
         try_files $uri $uri/ /index.html;
     }