feat(theme): a splash theme of our own, images in viewnior, a stop timeout for stasis

The Catppuccin splash showed the NixOS logo and the loading animation and
nothing else. The NixOS plymouth module links boot.plymouth.logo into every
theme directory named catppuccin-<flavour>, and the two-step module has no
alignment key for that header image, so its distance from the animation is not
the theme's to set. A theme under a name of its own is outside that loop.
moonarch-plymouth joins as a flake input; pkgs/moonarch-plymouth.nix rewrites
the descriptor's FHS ImageDir to the store path.

Images opened in Chromium although viewnior has been installed since 3e40a70:
no mimeapps.list named image/* at all, so xdg-open took the first candidate
from mimeinfo.cache. xdg.mime points gif, jpeg, png and webp at
viewnior.desktop. webp needs addedAssociations besides, which viewnior.desktop
does not declare, and webp-pixbuf-loader, without which gdk-pixbuf cannot
decode it.

stasis does not answer SIGTERM as part of the session teardown, and the default
90 seconds left the whole user manager waiting on it: niri received its own
SIGTERM only once stasis had been killed, 88 seconds in the journal.
TimeoutStopSec = 5. What it holds is an idle timer, so nothing is lost when it
goes down hard.

glab on the ThinkPad.

modules/services.nix

@@ -272,6 +272,12 @@ in
       ExecReload = "${lib.getExe pkgs.stasis} reload";
       Restart = "on-failure";
       RestartSec = 2;
+      # Stopped by hand it exits at once; stopped as part of the session it
+      # does not answer SIGTERM, and the default 90 seconds left the whole
+      # user manager waiting on it — niri received its own SIGTERM only once
+      # stasis had been killed. Measured at 88s in the journal. What it holds
+      # is an idle timer, so nothing is lost when it goes down hard.
+      TimeoutStopSec = 5;
       IPAddressDeny = "any";
       ProtectProc = "invisible";
       ProcSubset = "pid";