feat(vereinfacht): member form flash and show page

- Form: show Vereinfacht sync warning after save via SyncFlash
- Show: load API debug response; MembershipFees: contact ID, link, no-contact warning

lib/mv_web/live/member_live/form.ex

@@ -319,11 +319,40 @@ defmodule MvWeb.MemberLive.Form do
     socket =
       socket
       |> put_flash(:info, flash_message)
+      |> maybe_put_vereinfacht_sync_flash(member.id)
       |> push_navigate(to: return_path(socket.assigns.return_to, member))
 
     {:noreply, socket}
   end
 
+  defp maybe_put_vereinfacht_sync_flash(socket, member_id) do
+    case Mv.Vereinfacht.SyncFlash.take(to_string(member_id)) do
+      {:warning, message} ->
+        put_flash(socket, :warning, translate_vereinfacht_flash(message))
+
+      {:ok, _message} ->
+        # Optionally show sync success; for now we keep only the main success message
+        socket
+
+      nil ->
+        socket
+    end
+  end
+
+  defp translate_vereinfacht_flash(message) when is_binary(message) do
+    prefix = "Vereinfacht: "
+
+    if String.starts_with?(message, prefix) do
+      detail = message |> String.trim_leading(prefix) |> String.trim()
+
+      Gettext.dgettext(MvWeb.Gettext, "default", "Vereinfacht: %{detail}",
+        detail: Gettext.dgettext(MvWeb.Gettext, "default", detail)
+      )
+    else
+      Gettext.dgettext(MvWeb.Gettext, "default", message)
+    end
+  end
+
   defp handle_save_error(socket, form) do
     # Always show a flash message when save fails
     # Field-level validation errors are displayed in form fields, but flash provides additional feedback