Guard pause/resume against disabled backends; clean up CLI errors

- PauseBackend and ResumeBackend return an error (not bool) when the
  backend is disabled, preventing an inconsistent state where the
  health state says "paused" but enabled=false.
- DisableBackend and EnableBackend now log uniform backend-transition
  lines with from/to instead of separate backend-disable/backend-enable
  messages.
- CLI errors strip gRPC boilerplate ("rpc error: code = ... desc = ")
  and display the server message in red (when color is enabled). Both
  the interactive shell and one-shot mode use the same formatError path.

cmd/maglevc/main.go

@@ -18,7 +18,7 @@ import (
 
 func main() {
 	if err := run(); err != nil {
-		fmt.Fprintf(os.Stderr, "error: %v\n", err)
+		fmt.Fprintf(os.Stderr, "%s\n", formatError(err))
 		os.Exit(1)
 	}
 }