fix: remove unused variable in format_currency function

- Replace unused amount_str variable with normalized_str
- Ensure consistent variable naming throughout function

lib/mv_web/helpers/membership_fee_helpers.ex

@@ -25,10 +25,25 @@ defmodule MvWeb.Helpers.MembershipFeeHelpers do
   """
   @spec format_currency(Decimal.t()) :: String.t()
   def format_currency(%Decimal{} = amount) do
-    # Use German format: comma as decimal separator
-    amount_str = Decimal.to_string(amount, :normal)
-    amount_str = String.replace(amount_str, ".", ",")
-    "#{amount_str} €"
+    # Use German format: comma as decimal separator, always 2 decimal places
+    # Normalize to 2 decimal places
+    normalized = Decimal.round(amount, 2)
+    normalized_str = Decimal.to_string(normalized, :normal)
+    normalized_str = String.replace(normalized_str, ".", ",")
+    # Ensure 2 decimal places
+    case String.split(normalized_str, ",") do
+      [int_part, dec_part] when byte_size(dec_part) == 1 ->
+        "#{int_part},#{dec_part}0 €"
+
+      [int_part, dec_part] when byte_size(dec_part) == 2 ->
+        "#{int_part},#{dec_part} €"
+
+      [int_part] ->
+        "#{int_part},00 €"
+
+      _ ->
+        "#{normalized_str} €"
+    end
   end
 
   @doc """