feat(power): lower refresh rate on battery

[atimofeev]

Add a Power & Sleep setting that lowers eligible displays to 60 Hz on battery and restores their previous mode on AC power.

Skip displays without an available 60 Hz mode, displays with a single refresh-rate option, current modes at or below 60 Hz, and VRR-enabled outputs.

Apply niri changes at runtime through output commands and use wlr-output-management as fallback.

Closes #1203

[atimofeev]

Need help with additional testing, as I only have a single device with multiple refresh rates display (240/60).

Also, changes to DisplayService.qml might be overengineered.

[atimofeev]

Here's the first issue:

• When you restart dms being on battery, it thinks that configured refresh rate is 60hz and it never goes back to the preferred 240hz

Probably can be resolved with this logic:

• Get active monitor profile refresh rate and use it as baseline
• If monitor profiles are not configured, fall back to monitor preferred refresh rate
• Re-check these steps on profile or power source change

[atimofeev]

Implemented stateless transitions between 60hz and higher refresh rates.
Also improved logging to be more human-readable.