fix(admin): resolve Nginx redirect loop with alias and named location

Use alias to strip /admin/ prefix for file lookups and @admin_spa
named location for SPA fallback, preventing internal redirect cycle.

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docker/nginx/admin.conf

@@ -19,10 +19,15 @@ server {
     }
 
     location /admin/ {
-        try_files $uri $uri/ /admin/index.html;
+        alias /usr/share/nginx/html/;
+        try_files $uri $uri/ @admin_spa;
     }
 
-    location / {
+    location @admin_spa {
+        rewrite ^ /index.html break;
+    }
+
+    location = / {
         return 301 /admin/;
     }
 }