feat(gtk): theme GTK4 apps to match (import catppuccin gtk-4.0 CSS)

Set gtk.gtk4.theme = config.gtk.theme so any GTK4 app added later is
themed too. GTK4 ignores gtk-theme-name, but home-manager renders this as
an `@import` of the theme's gtk-4.0/gtk.css into ~/.config/gtk-4.0/gtk.css
-- which libadwaita honours, since that file overrides the named colours
it uses (window_bg_color/accent_bg_color/view_bg_color, verified present
in catppuccin-gtk's GTK4 stylesheet). Setting it explicitly (to the same
value as the legacy default) also silences the stateVersion<26.05
default-change warning. nemo (GTK3) is unaffected.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lyrathorpe/home/desktop.nix

@@ -2,7 +2,7 @@
 # Imported only on hosts that run Sway (MBP, T400, Mac Pro); never pulled onto
 # the headless WSL host. Login (and the Sway session launch) is handled by the
 # greetd/ReGreet greeter -- see ../swaywm.nix -- so there is no tty1 autostart.
-{ pkgs, ... }:
+{ pkgs, config, ... }:
 {
   imports = [
     ./sway.nix
@@ -27,6 +27,13 @@
   # design, so no separate prefer-dark hint is needed.
   gtk = {
     enable = true;
+    # Theme GTK4 apps too (for any added later). GTK4 ignores gtk-theme-name,
+    # but home-manager turns this into an `@import` of the theme's
+    # gtk-4.0/gtk.css into ~/.config/gtk-4.0/gtk.css -- which even libadwaita
+    # honours, since that file overrides the named colours it uses
+    # (window_bg_color, accent_bg_color, ...). Set explicitly (same value as the
+    # legacy default) so it also silences the stateVersion<26.05 warning.
+    gtk4.theme = config.gtk.theme;
     theme = {
       name = "catppuccin-mocha-blue-standard";
       package = pkgs.catppuccin-gtk.override {