refactor: fix credo warnings and format code

- Replace Enum.map/2 |> Enum.join/2 with Enum.map_join/3 for efficiency
- Refactor get_existing_form_values to reduce cyclomatic complexity
- Replace length/1 with Enum.empty?/1 for better performance
- Update gettext translations

lib/mv_web/live/membership_fee_settings_live.ex

@@ -44,7 +44,8 @@ defmodule MvWeb.MembershipFeeSettingsLive do
         Map.put(params, "include_joining_cycle", false)
       end
 
-    {:noreply, assign(socket, form: AshPhoenix.Form.validate(socket.assigns.form, normalized_params))}
+    {:noreply,
+     assign(socket, form: AshPhoenix.Form.validate(socket.assigns.form, normalized_params))}
   end
 
   def handle_event("save", %{"settings" => params}, socket) do