fix: prevent [No Host] panel errors behind CDN or missing Host header

Force nginx to pass the domain as Host, add PANEL_DOMAIN fallback in Flask,
and document that the admin panel must be accessed over HTTP not HTTPS.

Co-authored-by: Cursor <cursoragent@cursor.com>

server/nginx/acme.conf.template

@@ -20,10 +20,11 @@ server {
 __PANEL_ALLOW__
         proxy_pass http://127.0.0.1:5080/;
         proxy_http_version 1.1;
-        proxy_set_header Host $host;
+        proxy_set_header Host __DOMAIN__;
         proxy_set_header X-Real-IP $remote_addr;
         proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
         proxy_set_header X-Forwarded-Proto $scheme;
+        proxy_set_header X-Forwarded-Host __DOMAIN__;
         proxy_set_header X-Forwarded-Prefix __PANEL_PREFIX__;
     }
 }