fix: treat URL with only custom fields as valid in ?fields= mode

Consider visible custom fields in compute_final_field_selection so that
a link with only custom_field_X is not wrongly treated as invalid and
reverted to session/global. Add test for URL containing only custom field.

test/mv_web/member_live/index_field_visibility_test.exs

@@ -361,6 +361,18 @@ defmodule MvWeb.MemberLive.IndexFieldVisibilityTest do
       assert html =~ "Alice"
     end
 
+    test "URL with only custom field keeps custom field visible (no invalid fallback)", %{
+      conn: conn,
+      custom_field: custom_field
+    } do
+      conn = conn_with_oidc_user(conn)
+      id = custom_field.id
+      {:ok, _view, html} = live(conn, "/members?fields=custom_field_#{id}")
+
+      # Selection must not be treated as invalid; custom field column stays visible
+      assert html =~ "M001" or html =~ custom_field.name
+    end
+
     test "handles rapid toggling", %{conn: conn} do
       conn = conn_with_oidc_user(conn)
       {:ok, view, _html} = live(conn, "/members")