feat: implement server-wide management actions and enhance TUI dashboard

pkg/tui/messages.go

@@ -16,5 +16,13 @@ type MsgActionDone struct {
 // MsgLogsLoaded carries log lines for the selected agent.
 type MsgLogsLoaded struct{ Lines []string }
 
+// MsgServerActionDone reports the result of a server-wide bulk action.
+type MsgServerActionDone struct {
+	Action   string
+	Total    int
+	Failed   int
+	Errors   []string
+}
+
 // MsgTick triggers a periodic refresh.
 type MsgTick struct{}