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/helpers/membership_fee_helpers.ex

@@ -28,6 +28,7 @@ defmodule MvWeb.Helpers.MembershipFeeHelpers do
     # Use German format: comma as decimal separator, always 2 decimal places
     # Normalize to 2 decimal places
     normalized = Decimal.round(amount, 2)
+
     normalized_str =
       normalized
       |> Decimal.to_string(:normal)