Nix/module: add defaultSettings

[fufexan]

Describe your PR, what does it fix/add?

TO BE MERGED AFTER #9930

Add defaultSettings option in the NixOS module. Can be disabled by setting it to {}.

Default settings currently includes allowing screencopy for XDPH and grim, and optionally adding exec-once = uwsm finalize in case withUWSM = true.

Is there anything you want to mention? (unchecked code, possible bugs, found problems, breaking compatibility, etc.)

Brainstorming is welcome. This is what made sense to me at the time of opening this PR.

Is it ready for merging, or does it need work?

Untested, no.

[fxzzi]

Are you sure we need uwsm finalize? According to the uwsm docs it states:

TLDR; if your compositor puts WAYLAND_DISPLAY (and along with it DISPLAY, or other important or useful variables) into systemd activation environment, uwsm will make everything work automagically, proceed to section 3.

Otherwise configure compositor to run uwsm finalize command at the end of its startup. It will deal with putting WAYLAND_DISPLAY and DISPLAY (if set) variables into activation environments in the best possible ways and signal unit readiness to systemd.

I am assuming hyprland does set these as uwsm wants and therefore the manual finalize shouldn't be needed

I am assuming this since I don't use this finalize command and I haven't faced any issues regarding unset WAYLAND_DISPLAY or DISPLAY, let me know if I'm incorrect here though.

[fufexan]

It may be a remnant from the early days of uwsm support, as it was needed sometimes to properly finish setting up the session.

If it's no longer needed, I'll remove it.

[fufexan]

Another possible approach would be to have a toggle enableDefaultConfig explaining what settings are being added, since it's something you either want or don't want.

But I believe this would hurt visibility, as having the option an attrset directly shows its value as default.