fix: audit fix — avoid latent stdout pipe deadlock in run_command (v0.8.3)

Piping stdout without draining while blocking in child.wait() risks deadlock
if a command writes more than one OS pipe buffer (~64 KB on Linux). Current
callers (systemctl, niri msg, loginctl) stay well under that, but the
structure was fragile. stdout is now discarded; stderr continues to be
captured for error reporting.

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "moonset"
-version = "0.8.2"
+version = "0.8.3"
 edition = "2024"
 description = "Wayland session power menu with GTK4 and Layer Shell"
 license = "MIT"