fix(membership): add chronological sort key for custom :date fields

Custom :date values are real Date structs; sorting them by Erlang term
order compares day, then month, then year, so the member list ordered
them like day-first text instead of chronologically. Derive the sort key
from a single shared helper that maps a date to its Gregorian day count,
leaving the other value types at their already-correct natural order.

lib/mv/membership/custom_field_sort.ex

@@ -0,0 +1,30 @@
+defmodule Mv.Membership.CustomFieldSort do
+  @moduledoc """
+  Derives a term-order-comparable sort key from a custom-field value.
+
+  Custom-field values are stored in an Ash `:union` attribute, so a value
+  arrives either as an `%Ash.Union{}` or as its already-unwrapped term. This
+  module is the single source of truth for turning such a value into a key that
+  `Enum.sort_by/2` can order correctly per `value_type`.
+
+  nil / empty handling is intentionally NOT this function's concern — the call
+  sites split present from absent values before sorting.
+  """
+
+  @doc """
+  Returns a term-order-comparable sort key for `value`, given its `value_type`.
+
+  For every non-date type the natural unwrapped value is returned, so
+  `:integer` sorts numerically and `:string` / `:email` sort lexicographically.
+  """
+  def sort_key(%Ash.Union{value: value, type: type}, _expected_type),
+    do: sort_key(value, type)
+
+  def sort_key(value, :string) when is_binary(value), do: value
+  def sort_key(value, :integer) when is_integer(value), do: value
+  def sort_key(value, :boolean) when is_boolean(value), do: value
+  # Gregorian day count is a chronological integer key (earlier date ⇒ smaller).
+  def sort_key(%Date{} = date, :date), do: Date.to_gregorian_days(date)
+  def sort_key(value, :email) when is_binary(value), do: value
+  def sort_key(value, _type), do: to_string(value)
+end