fix(nginx): drop trailing slash on variable ui proxy_pass

With a variable upstream ($ui_upstream, added for docker-DNS re-resolution),
a trailing-slash URI in proxy_pass makes nginx forward '/' to the UI for every
sub-path, so /_next/static/*.css and /i/<invoice> all return the app index HTML
(200 text/html) -> the UI renders unstyled and client routing breaks. Omit the
URI to preserve the full request path, matching the /api/ block.

nginx/conf.d/default.conf

@@ -57,7 +57,12 @@ server {
 
   location / {
     set $ui_upstream "ui";
-    proxy_pass http://$ui_upstream:3000/;
+    # No trailing-slash URI: with a VARIABLE upstream, a trailing "/" makes
+    # nginx send "/" to the UI for every sub-path (assets, /i/<invoice>), so
+    # the app's index HTML is returned for /_next/static/*.css (200 text/html)
+    # → the whole UI renders unstyled and client routing dies. Omit the URI so
+    # the full request path is preserved, exactly like the /api/ block above.
+    proxy_pass http://$ui_upstream:3000;
     proxy_http_version 1.1;
     proxy_set_header Host $host;
     proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;