flood-for-transmission: add floodSettings parameter

https://github.com/johman10/flood-for-transmission/xull/540 added
support for setting default values for the UI config options in
config.json. Add a `floodSettings` parameter to the derivation to
support generation of config.json in nixpkgs.

pkgs/by-name/fl/flood-for-transmission/package.nix

@@ -1,8 +1,14 @@
 # To use this package, use: `services.transmission.webHome = pkgs.flood-for-transmission;`
+# The configuration [1] can be modified by overriding floodSettings:
+# pkgs.flood-for-transmission.override {
+#   floodSettings.SWITCH_COLORS = true;
+# };
+# [1]: https://github.com/johman10/flood-for-transmission?tab=readme-ov-file#beta-customization
 {
   lib,
   buildNpmPackage,
   fetchFromGitHub,
+  floodSettings ? null,
 }:
 
 buildNpmPackage rec {
@@ -24,10 +30,15 @@ buildNpmPackage rec {
     runHook preInstall
 
     cp -r public $out
+    rm $out/config.json.defaults
 
     runHook postInstall
   '';
 
+  postInstall = lib.optionalString (floodSettings != null) ''
+    echo '${builtins.toJSON floodSettings}' > $out/config.json
+  '';
+
   meta = with lib; {
     description = "Flood clone for Transmission";
     homepage = "https://github.com/johman10/flood-for-transmission";