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/desktop.nix

@@ -193,6 +193,18 @@ in
     galculator
   ];
 
+  xdg.mime = {
+    defaultApplications = {
+      "image/gif" = "viewnior.desktop";
+      "image/jpeg" = "viewnior.desktop";
+      "image/png" = "viewnior.desktop";
+      "image/webp" = "viewnior.desktop";
+    };
+    addedAssociations."image/webp" = "viewnior.desktop";
+  };
+
+  programs.gdk-pixbuf.modulePackages = [ pkgs.webp-pixbuf-loader ];
+
   # Polkit agent for privilege prompts inside the session.
   security.polkit.enable = true;
   # The setuid pkexec wrapper is opt-in and off by default. Without it pkexec

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";

modules/theme.nix

@@ -47,11 +47,6 @@ let
   # it, so the link target below cannot drift from what Kvantum looks up.
   kvantumThemeName = "catppuccin-mocha-lavender";
 
-  # Mocha like everything else here. The package installs under
-  # share/plymouth/themes/catppuccin-${variant}, which is the name boot.plymouth
-  # asks for. Upstream Catppuccin has no accent variants for Plymouth.
-  plymouthVariant = "mocha";
-  plymouthTheme = pkgs.catppuccin-plymouth.override { variant = plymouthVariant; };
 in
 {
   environment.etc = {
@@ -119,10 +114,13 @@ in
   # With systemd in the initrd, which this configuration uses, systemd asks for
   # the LUKS passphrase and Plymouth draws that prompt; the script initrd instead
   # shows it as text before the splash comes up.
+  # The theme carries a name of its own, outside the catppuccin-<flavour> set
+  # that the NixOS plymouth module links its logo into, so the splash shows
+  # what the theme brings and nothing besides.
   boot.plymouth = {
     enable = true;
-    theme = "catppuccin-${plymouthVariant}";
-    themePackages = [ plymouthTheme ];
+    theme = "moonarch";
+    themePackages = [ moonarchPkgs.moonarch-plymouth ];
   };
 
   # Qt applications are wrapped, so each one looks for plugins in its own