HiDPI / fractional scaling support per-monitor

[oskardotglobal]

Discussed in #824

^{Originally posted by mlltAero November 19, 2025}
Currently, WinApps uses a global RDP_SCALE for all RemoteApp windows. On HiDPI laptops, this causes RemoteApps to render tiny because the scale does not automatically match the display’s effective scaling.

Problem:

RDP_SCALE is static and does not account for fractional scaling in GNOME or per-monitor DPI.
Users with HiDPI laptop screens must manually adjust RDP_SCALE in winapps.conf.

Proposed feature:

Automatically detect the primary monitor and its scaling (e.g., GNOME fractional scaling or via xrandr resolution vs. native panel size) and convert to supported scaling factor (nearest neighbor).
Adjust RDP_SCALE for RemoteApp windows on launch so they render correctly on HiDPI displays.