Document -urlPrefix flag for subdirectory deployment

Admin-UI.md

@@ -32,6 +32,7 @@ weed admin [options]
 | `-adminPassword` | "" | Admin interface password (if empty, auth is disabled) |
 | `-readOnlyUser` | "" | Read-only user username (optional) |
 | `-readOnlyPassword` | "" | Read-only user password (optional; requires adminPassword) |
+| `-urlPrefix` | "" | URL path prefix for subdirectory deployment (e.g. `/seaweedfs`) |
 
 ## Examples
 
@@ -58,6 +59,27 @@ weed admin -port=23646 -masters="localhost:9333" -dataDir="~/seaweedfs-admin"
 weed admin -adminUser=admin -adminPassword=secret123 -masters="localhost:9333"
 ```
 
+### Behind a Reverse Proxy (Subdirectory)
+
+```bash
+# Serve the admin UI under /seaweedfs/ subdirectory
+weed admin -masters="localhost:9333" -urlPrefix=/seaweedfs
+```
+
+With this configuration, the admin UI is accessible at `http://localhost:23646/seaweedfs/admin`. All static assets, API endpoints, navigation links, and session cookies are automatically scoped to the prefix.
+
+Example nginx reverse proxy configuration:
+
+```nginx
+location /seaweedfs/ {
+    proxy_pass http://localhost:23646/seaweedfs/;
+    proxy_set_header Host $host;
+    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;
+}
+```
+
 ### Production Deployment
 
 ```bash
@@ -275,6 +297,11 @@ Or with custom port:
 http://localhost:PORT
 ```
 
+If `-urlPrefix` is configured, include the prefix in the URL:
+```
+http://localhost:23646/seaweedfs/admin
+```
+
 ### Dashboard (/)
 
 The main dashboard provides a comprehensive overview of your SeaweedFS cluster: