feat(admin): add -urlPrefix flag for subdirectory deployment (#8670)

Allow the admin server to run behind a reverse proxy under a
subdirectory by adding a -urlPrefix flag (e.g. -urlPrefix=/seaweedfs).

Closes #8646

weed/admin/view/app/cluster_volume_servers.templ

@@ -337,7 +337,7 @@ templ ClusterVolumeServers(data dash.ClusterVolumeServersData) {
             '<a href="http://' + (server.publicUrl || server.address) + '/ui/index.html" target="_blank" class="btn btn-outline-primary">' +
             '<i class="fas fa-external-link-alt me-1"></i>Open Volume Server UI' +
             '</a>' +
-            '<a href="/storage/volumes?server=' + encodeURIComponent(server.address) + '" class="btn btn-outline-info">' +
+            '<a href="' + (window.__BASE_PATH__ || '') + '/storage/volumes?server=' + encodeURIComponent(server.address) + '" class="btn btn-outline-info">' +
             '<i class="fas fa-database me-1"></i>View Volumes' +
             '</a>' +
             '</div>' +