gnome: route headerbar CSS through stylix.targets.gtk.extraCss

Plain gtk.gtk{3,4}.extraCss is silently dropped once stylix owns the
theme — stylix prints an evaluation warning and writes nothing. Move
the headerbar-shrink customisations to stylix.targets.gtk.extraCss so
they actually apply.

Also switch qt.platformTheme from the deprecated "gnome" to "adwaita".

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

settings/gnome.nix

@@ -49,7 +49,7 @@
 
     qt = {
       enable = true;
-      platformTheme = "gnome";
+      platformTheme = "adwaita";
       style = "adwaita-dark";
     };
 
@@ -60,13 +60,10 @@
     home-manager.users.fred = { config, lib, pkgs, ... }: {
       # Minimal titlebars — stylix manages the GTK theme; we layer our
       # headerbar shrink on top via programs.gtk.*.extraCss.
-      gtk = {
-        enable = true;
-        gtk3.extraCss = ''
-          headerbar { min-height: 0; padding: 0; margin: 0; }
-          headerbar .title { font-size: 0; }
-        '';
-        gtk4.extraCss = ''
+      gtk.enable = true;
+      # gtk.gtk3.extraCss / gtk4.extraCss are no-ops once stylix owns the
+      # theme — extra CSS has to go through stylix.targets.gtk.extraCss.
+      stylix.targets.gtk.extraCss = ''
         headerbar { min-height: 0; padding: 0; margin: 0; }
         headerbar .title { font-size: 0; }
         window:backdrop {
@@ -78,7 +75,6 @@
           color: @headerbar_fg_color;
         }
       '';
-      };
 
       # GNOME dconf settings — stylix owns colour-scheme, gtk-theme,
       # cursor-theme and accent-color now.