Route fontconfig through fonts.fontconfig.localConf

/etc/fonts/conf.d is owned by the NixOS fontconfig module and rejects
environment.etc entries. Same content as
defaults/etc/fonts/conf.d/65-moonarch-fonts.conf.

modules/desktop.nix

@@ -52,12 +52,32 @@ in
 
     # Configs outside /etc/xdg/ in the Arch package as well.
     "mpv/mpv.conf".source = "${etc}/mpv/mpv.conf";
-    "fonts/conf.d/65-moonarch-fonts.conf".source =
-      "${etc}/fonts/conf.d/65-moonarch-fonts.conf";
     "xdg-desktop-portal/niri-portals.conf".source =
       "${etc}/xdg-desktop-portal/niri-portals.conf";
   };
 
+  # Generic-family defaults. Same content as
+  # defaults/etc/fonts/conf.d/65-moonarch-fonts.conf, but routed through the
+  # fontconfig module: /etc/fonts/conf.d/ is owned by NixOS and rejects
+  # environment.etc entries.
+  fonts.fontconfig.localConf = ''
+    <description>Moonarch generic-family defaults</description>
+    <!-- binding="strong" required: the default weak <prefer> ranks behind
+         stock generic fallbacks, so it would not take effect. -->
+    <alias binding="strong">
+      <family>sans-serif</family>
+      <prefer>
+        <family>UbuntuSans Nerd Font</family>
+      </prefer>
+    </alias>
+    <alias binding="strong">
+      <family>monospace</family>
+      <prefer>
+        <family>UbuntuSansMono Nerd Font</family>
+      </prefer>
+    </alias>
+  '';
+
   # --- Compositor ---
   programs.niri.enable = true;
 
@@ -137,7 +157,7 @@ in
     colloid-icon-theme
     libsForQt5.qtstyleplugin-kvantum
     qt6Packages.qtstyleplugin-kvantum
-    qt6ct
+    qt6Packages.qt6ct
     qt6.qt5compat
 
     # CLI tools referenced by zshrc and the helper scripts