refactor(web): extract format_ash_error to MvWeb.ErrorHelpers

Use shared ErrorHelpers in UserLive.Index for consistent Ash error formatting.

lib/mv_web/error_helpers.ex

@@ -0,0 +1,25 @@
+defmodule MvWeb.ErrorHelpers do
+  @moduledoc """
+  Shared helpers for formatting errors in the web layer.
+
+  Use `format_ash_error/1` for Ash errors so behaviour stays consistent
+  (e.g. handling Invalid errors whose entries may lack a `:message` field).
+  """
+
+  @doc """
+  Formats an Ash error for display to the user.
+
+  Handles `Ash.Error.Invalid` by joining error messages; entries without
+  a `:message` field are inspected to avoid FunctionClauseError.
+  Other errors are inspected.
+  """
+  @spec format_ash_error(Ash.Error.t() | term()) :: String.t()
+  def format_ash_error(%Ash.Error.Invalid{errors: errors}) when is_list(errors) do
+    Enum.map_join(errors, ", ", fn
+      %{message: message} when is_binary(message) -> message
+      other -> inspect(other)
+    end)
+  end
+
+  def format_ash_error(error), do: inspect(error)
+end