refactor: improve format_currency robustness and reduce complexity

Extract formatting logic into helper functions to reduce cyclomatic complexity. Improve pattern matching for edge cases.

lib/mv_web/helpers/membership_fee_helpers.ex

@@ -26,30 +26,42 @@ 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, always 2 decimal places
-    # Normalize to 2 decimal places
     normalized = Decimal.round(amount, 2)
+    normalized_str = Decimal.to_string(normalized, :normal)
 
-    normalized_str =
-      normalized
-      |> Decimal.to_string(:normal)
-      |> String.replace(".", ",")
+    format_currency_parts(normalized_str)
+  end
 
-    # 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} €"
+  # Formats currency string with comma as decimal separator
+  defp format_currency_parts(normalized_str) do
+    case String.split(normalized_str, ".") do
+      [int_part, dec_part] ->
+        format_with_decimal_part(int_part, dec_part)
 
       [int_part] ->
         "#{int_part},00 €"
 
       _ ->
-        "#{normalized_str} €"
+        # Fallback for unexpected split results
+        "#{String.replace(normalized_str, ".", ",")} €"
     end
   end
 
+  # Formats currency with decimal part, ensuring exactly 2 decimal places
+  defp format_with_decimal_part(int_part, dec_part) do
+    dec_size = byte_size(dec_part)
+
+    formatted_dec =
+      cond do
+        dec_size == 1 -> "#{dec_part}0"
+        dec_size == 2 -> dec_part
+        dec_size > 2 -> String.slice(dec_part, 0, 2)
+        true -> "00"
+      end
+
+    "#{int_part},#{formatted_dec} €"
+  end
+
   @doc """
   Formats an interval atom as a translated string.
 
@@ -179,6 +191,7 @@ defmodule MvWeb.Helpers.MembershipFeeHelpers do
         |> Enum.filter(fn cycle ->
           CalendarCycles.current_cycle?(cycle.cycle_start, fee_type.interval, today)
         end)
+        |> Enum.sort_by(& &1.cycle_start, {:desc, Date})
         |> List.first()
     end
   end