[Feature] D-Bus Interface like PPD

First of all, thank you for developing `auto-cpufreq` — it’s an excellent tool for dynamic CPU frequency scaling.

Currently, several desktop environments (DEs) provide a **power mode/slider integration** in their system tray or settings panels. These sliders usually work with `power-profiles-daemon` or `tuned-ppd`, allowing users to switch between modes such as *Performance*, *Balanced*, and *Power Saver*.

It would be great if `auto-cpufreq` could integrate with these power sliders across DEs.

## Benefits
- Unified user experience across desktop environments.
- Quick and intuitive switching between `auto-cpufreq` profiles without relying on CLI.
- Encourages wider adoption since many users prefer GUI-driven controls.

## Examples of DE Integration Points
- **KDE Plasma** – System Tray Power Management Slider (via `powerdevil`).
- **GNOME** – Settings → Power Mode toggle (exposed via `power-profiles-daemon`).
- **XFCE** – Power Manager panel plugin.
- **Cinnamon / MATE / Budgie** – Power applets that already use PPD as backend.

## Possible Implementation Approaches
- Allow `auto-cpufreq` to **expose its profiles via D-Bus**, making them visible to DEs in the same way as `power-profiles-daemon`.
- Provide an **optional compatibility layer** that lets DEs treat `auto-cpufreq` as if it were `power-profiles-daemon` or `tuned-ppd`.
- Offer a **configurable mapping** of `auto-cpufreq` modes to standard power profiles.

## References
- KDE Plasma PowerDevil: [https://invent.kde.org/plasma/powerdevil](https://invent.kde.org/plasma/powerdevil)
- GNOME Power Profiles: [https://gitlab.freedesktop.org/upower/power-profiles-daemon](https://gitlab.freedesktop.org/upower/power-profiles-daemon)

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### **Feature Request Summary**
Add integration between `auto-cpufreq` and desktop environment power sliders (KDE, GNOME, XFCE, etc.) by exposing profiles through a standard interface (e.g., D-Bus, PPD compatibility), similar to how `power-profiles-daemon` and `tuned-ppd` work. I just really like the look and feel of KDE's powerdevil

Start by reviewing the referenced KDE PowerDevil and power-profiles-daemon interfaces, then compare the proposed D-Bus, compatibility-layer, and profile-mapping approaches. Done should mean that auto-cpufreq profiles are exposed through a standard interface and can be switched from desktop power sliders across the stated environments.

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