Does your feature request involve difficulty for you to complete a task? Please describe.

On Linux, readest only enables client-side decorations (CSD) based on the OS type, but not on the desktop environment.
This works fine on GNOME, but on KDE (Plasma) windows look broken or lack proper decorations.

Describe the solution you'd like

I would like readest to detect the current desktop environment in addition to the OS, so that CSD can be enabled automatically for KDE/Plasma as well. For example, it could check XDG_CURRENT_DESKTOP or DESKTOP_SESSION and enable CSD for both GNOME and KDE.

Describe alternatives you've considered

Another option would be to provide an explicit user setting to force client-side decorations regardless of OS/DE detection.

Additional context

Without this, readest windows look inconsistent with the rest of the KDE environment.
Here is a screenshot showing the window border issue on KDE:

Ref: #196 #1551