feat(member): let members tailor overview density and visible columns

The View dropdown drives row density and whether the Member and Address
fields render as composite cells or split into their underlying columns;
the column manager toggles visibility and resets to the curated default.
All choices persist per browser through the URL/session/cookie/global
chain, so a saved layout survives reloads without a per-account store.

test/mv_web/member_live/index_column_manager_test.exs

@@ -0,0 +1,76 @@
+defmodule MvWeb.MemberLive.IndexColumnManagerTest do
+  @moduledoc """
+  §1.6 — The column manager sits in the filter toolbar, offers a per-column
+  visibility toggle plus All and None controls styled as buttons (not links),
+  and applies changes while preserving the existing persistence chain.
+  """
+  use MvWeb.ConnCase, async: false
+
+  import Phoenix.LiveViewTest
+
+  alias Mv.Helpers.SystemActor
+
+  setup %{conn: conn} do
+    {:ok, _} =
+      Mv.Membership.create_member(
+        %{first_name: "Manager", last_name: "Columns", email: "manager@example.com"},
+        actor: SystemActor.get_system_actor()
+      )
+
+    %{conn: conn_with_oidc_user(conn)}
+  end
+
+  test "column manager trigger sits alongside the filter controls", %{conn: conn} do
+    {:ok, view, _html} = live(conn, ~p"/members")
+
+    # Both the filter panel trigger and the column-manager trigger render in the
+    # same toolbar.
+    assert has_element?(view, ~s(button[aria-label="Filter members"]))
+    assert has_element?(view, "button[aria-controls='field-visibility-menu']")
+  end
+
+  test "All and None are buttons, not links, and per-column toggles are checkboxes", %{conn: conn} do
+    {:ok, view, _html} = live(conn, ~p"/members")
+
+    view |> element("button[aria-controls='field-visibility-menu']") |> render_click()
+
+    # All / None render as buttons with button styling.
+    assert has_element?(view, "button.btn[phx-click='select_all']", "All")
+    assert has_element?(view, "button.btn[phx-click='select_none']", "None")
+    refute has_element?(view, "a[phx-click='select_all']")
+    refute has_element?(view, "a[phx-click='select_none']")
+
+    # Per-column visibility toggles are accessible checkbox menu items.
+    assert has_element?(view, "button[role='menuitemcheckbox'][phx-value-item='name']")
+  end
+
+  test "toggling a column applies and is reflected in the URL (persistence)", %{conn: conn} do
+    {:ok, view, _html} = live(conn, ~p"/members")
+
+    view |> element("button[aria-controls='field-visibility-menu']") |> render_click()
+    view |> element("button[phx-value-item='join_date']") |> render_click()
+
+    # The selection is pushed to the URL fields param so it survives reloads.
+    path = assert_patch(view)
+    assert path =~ "fields="
+    refute has_element?(view, "[data-testid='join_date']")
+  end
+
+  test "a reset button restores the curated default column set", %{conn: conn} do
+    {:ok, view, _html} = live(conn, ~p"/members")
+
+    view |> element("button[aria-controls='field-visibility-menu']") |> render_click()
+
+    # Reset is a button styled like All/None, sitting next to them, with a tooltip.
+    assert has_element?(view, "button.btn[phx-click='reset_fields']")
+
+    # Hide a curated-default column, then reset restores it.
+    view |> element("button[phx-value-item='join_date']") |> render_click()
+    assert_patch(view)
+    refute has_element?(view, "[data-testid='join_date']")
+
+    view |> element("button[phx-click='reset_fields']") |> render_click()
+    assert_patch(view)
+    assert has_element?(view, "[data-testid='join_date']")
+  end
+end