feat(downstream): add easy configuration overrides

- Introduce nomarchy.configOverrides option to map a user directory to ~/.config
- Implement automatic merging of upstream defaults and user overrides
- Use lib.mkDefault for all upstream mappings to allow granular HM overrides
- Update installer template with usage examples

installer/install-nomarchy.sh

@@ -254,8 +254,11 @@ cat <<EOF > /mnt/etc/nixos/home.nix
   ];
 
   # Example: How to override Nomarchy defaults
-  # nomarchy.home.terminal = "kitty";
-  # nomarchy.home.themeOverride = "catppuccin-mocha";
+  # nomarchy.fonts.monospace = "FiraCode Nerd Font";
+  
+  # Example: Easy configuration overrides
+  # Place your custom configs in an 'overrides' folder next to home.nix
+  # nomarchy.configOverrides = ./overrides;
 }
 EOF