fix stale SERVER_URL when changing env var in Docker (#1714)

settings.js (which injects SERVER_URL into the browser) was served
without Cache-Control headers, causing Cloudflare and other reverse
proxies to cache the old value indefinitely. Additionally, when
SERVER_LOCK is enabled, the persisted server URL in localStorage was
never compared against the current window.SERVER_URL, so same-browser
sessions kept using the old server even after settings.js was updated.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

ng.conf.template

@@ -20,9 +20,11 @@ server {
 
   location ${PUBLIC_PATH}settings.js {
     alias /etc/nginx/conf.d/settings.js;
+    add_header Cache-Control "no-store";
   }
 
   location ${PUBLIC_PATH}/settings.js {
     alias /etc/nginx/conf.d/settings.js;
+    add_header Cache-Control "no-store";
   }
 }