fix: audit LOW fixes — nmcli guards, sink cancel, cpugov stderr, gpu interval

- moonarch-vpn: add `--` argument-terminator to `nmcli connection up/down`
  so a profile name starting with `-` is never interpreted as a flag.
- moonarch-sink-switcher: guard against empty `$sink` when walker is
  cancelled, since awk masks walker's non-zero exit. Prevents the error
  `pactl set-default-sink ""` on every dismissal.
- moonarch-waybar-cpugov: redirect stderr so non-cpufreq systems (VMs,
  some desktops) do not spam the journal on every 60s poll.
- waybar config: switch custom/gpu-usage from `restart-interval: 10` to
  `interval: 60`. The module lives in a closed drawer, a 10 s poll spawn
  was unnecessary background noise.

defaults/xdg/waybar/config

@@ -319,7 +319,7 @@
     "custom/gpu-usage": {
         "exec": "moonarch-waybar-gpustat",
         "return-type": "json",
-        "restart-interval": 10
+        "interval": 60
     },
     "battery": {
         "bat": "BAT0",