Support per-power-source profile overrides in omarchy-powerprofiles-set

[fase29]

What

Adds an optional user config file at ~/.config/omarchy/power-profile that lets
you choose which power profile is applied on battery vs. on AC:

battery=power-saver
ac=balanced

Each key is looked up against the profiles reported by powerprofilesctl list,
so any valid profile name works.

Why

The udev rule installed by powerprofilesctl-rules.sh runs
omarchy-powerprofiles-set on every AC/USB power event, and the script
unconditionally forces performance whenever the machine is on AC.

On my laptop that's a problem: performance makes the chassis run hot enough
that the built-in keyboard becomes uncomfortable to type on. I want to stay on
power-saver (or balanced) even while plugged in, but there was no way to do
that short of editing the script directly — which git pull reverts on the next
update.

This makes the AC/battery profile targets configurable, so the tradeoff between
performance and heat/noise/battery can be set to preference instead of being
hardcoded.

Behavior

• Fully backward compatible. With no config file present, behavior is
  identical to today: performance on AC (falling back to balanced if
  unavailable), balanced on battery.
• Unknown keys or invalid/unavailable profile values fall back to the built-in
  defaults, so a typo can never leave the machine in a broken state.
• The user's home is derived from the script's own path rather than $HOME,
  because udev invokes the script as root via systemd-run.

Example

~/.config/omarchy/power-profile

battery=power-saver
ac=power-saver

Keeps the laptop cool and quiet regardless of whether it's plugged in.

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