fix(members): restore column visibility from URL on reload

Read 'fields' from URI when conn.params has no query (e.g. full page load).
When ?fields=... is present use URL-only selection so columns are not
merged with global settings. Fall back to session+global when URL has
only invalid field names.

lib/mv_web/live/member_live/index/field_visibility.ex

@@ -64,6 +64,25 @@ defmodule MvWeb.MemberLive.Index.FieldVisibility do
     member_fields ++ custom_field_names
   end
 
+  @doc """
+  Builds field selection from URL only: fields in `url_selection` are visible, all others false.
+  Use when `?fields=...` is in the URL so column visibility is not merged with global settings.
+  """
+  @spec selection_from_url_only(%{String.t() => boolean()}, [struct()]) :: %{
+          String.t() => boolean()
+        }
+  def selection_from_url_only(url_selection, custom_fields) when is_map(url_selection) do
+    all_fields = get_all_available_fields(custom_fields)
+
+    Enum.reduce(all_fields, %{}, fn field, acc ->
+      field_string = field_to_string(field)
+      visible = Map.get(url_selection, field_string, false)
+      Map.put(acc, field_string, visible)
+    end)
+  end
+
+  def selection_from_url_only(_, _), do: %{}
+
   @doc """
   Merges user field selection with global settings.