fix(nginx): revalidate index.html and translations on every load

i18n/*.json and index.html keep their names across deploys, so browsers kept old files and showed raw keys like NAV.DASHBOARD. Hashed /assets/ now get a one-year immutable cache.

nginx.conf

@@ -1,9 +1,17 @@
 server {
     listen 80;
-
-    location / {
     root /usr/share/nginx/html;
+
+    # Vite puts a content hash in these file names, so they never change.
+    location /assets/ {
+        add_header Cache-Control "public, max-age=31536000, immutable";
+    }
+
+    # index.html and /i18n/*.json keep their names across deploys. Without
+    # no-cache the browser shows old translation keys or loads an old bundle.
+    location / {
         index index.html index.htm;
+        add_header Cache-Control "no-cache";
 
         # Handle React Router navigation
         try_files $uri $uri/ /index.html;