The Window Controls Overlay API applies a transparent overlay on
Windows, but not on Linux. This PR addresses this by capturing the area
underneath the overlay, averaging the color of the area, and setting
this as the overlay background color.
It will also detect whether to make the control symbols white or black,
depending on how dark the background color is.
On Linux, this will set both the background color and the symbol color,
on Windows it will just set the symbol color.
<img width="721" alt="image"
src="https://github.com/user-attachments/assets/e6f9f8f8-a49f-41b6-984e-09e7d52c631d">
This makes it possible to send wsh commands from wsh on a remote session
to wavesrv running locally. The exact behavior of running those commands
isn't implemented, but the underlying interface is added here.
This PR implements the [Window Controls Overlay
API](https://web.dev/articles/window-controls-overlay) to let us hide
the menu bar on Windows and Linux and directly embed the window controls
in our tab bar. With #239 merged, we no longer need the menu bar on
these platforms.
The overlaid window controls are transparent so they will take on the
background from the app. I've updated the tab bar to flow properly using
the API's CSS environment variables.
At some point, we may want to update the logic around the symbolColor so
that it can ensure a proper contrast between the background and the
symbols in the window controls. For now, setting them to white works for
all the backgrounds we currently support.

https://github.com/user-attachments/assets/8d19b512-5281-42b9-8abb-ccb9b850061f