Fix the Servers page failing when no servers are configured

An empty server list was sent as null, so the page threw on .length and
showed only the loading placeholder. The API now always returns a list,
the page tolerates null, and tests cover the empty config and the token.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

internal/admin/admin.go

@@ -71,7 +71,7 @@ func (a *Admin) overview(w http.ResponseWriter, r *http.Request) {
 	for _, s := range snap.Servers {
 		stats[s.Name] = s
 	}
-	var servers []serverView
+	servers := []serverView{}
 	for _, s := range a.Cfg.Current().Servers {
 		v := serverView{Server: s}
 		if h, ok := states[s.Name]; ok {