Fix diag redirects pointing at internal :7443 (unreachable by browser)

The panel vhost listens on 7443 behind the SNI stream (public port is
443). nginx builds absolute redirect Location headers from the server's
listen port, so `return 302 /path` in the diag SSO bridge emitted
`Location: https://<domain>:7443/...`. Browsers followed that to port
7443, which UFW blocks (only 22/80/443 open) → the diagnostics page
hung and never loaded.

Set `absolute_redirect off` on the panel vhost so nginx emits relative
Location headers; the browser resolves them against the real origin
(:443). Verified with nginx: listen 7443 + return 302 goes from
`http://host:7443/panelXXXX/` to a bare `/panelXXXX/`.

This was the actual cause of the "diagnostics page doesn't load after
panel login" report; earlier fixes addressed real but secondary issues
on the same path.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

x-ui-latest.sh

@@ -457,6 +457,10 @@ server {
     root /var/www/html/;
     real_ip_header proxy_protocol;
     set_real_ip_from 127.0.0.1;
+    # This vhost listens on 7443 behind the SNI stream (public port 443). Without
+    # this, nginx bakes :7443 into redirect Location headers (return/error_page),
+    # so browsers get sent to an unreachable port. Keep redirects relative.
+    absolute_redirect off;
     # Larger h2 preread window improves single-stream upload throughput
     http2_body_preread_size 128k;
     client_body_buffer_size 512k;