feat(ledstrip): switch the strip on and off with the session

`udt-ledstrip on|off` switches the strip, replaying the gradient when it
comes on; with no argument it still only replays the gradient, so a
wallpaper change leaves a strip the user switched off alone.

The last address is cached in ~/.cache/udt/ledstrip.json, because `off`
runs as the session ends and cannot wait up to 20 s for a broadcast.
From the cache a switch takes under half a second; discovery is the
fallback when the cached address fails.

hypr-theme calls it at login, on Hyprland's shutdown event, and around
suspend from hypridle.

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

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