Disable board edit buttons for select all/none when not privileged

ref: https://github.com/basecamp/fizzy/discussions/1992

test/controllers/boards_controller_test.rb

@@ -145,4 +145,25 @@ class BoardsControllerTest < ActionDispatch::IntegrationTest
 
     assert_response :forbidden
   end
+
+  test "disables select all/none buttons for non-privileged user" do
+    logout_and_sign_in_as :jz
+    assert_not users(:jz).can_administer_board?(boards(:writebook))
+
+    get edit_board_path(boards(:writebook))
+
+    assert_response :success
+    assert_select "button[disabled]", text: "Select all"
+    assert_select "button[disabled]", text: "Select none"
+  end
+
+  test "enables select all/none buttons for privileged user" do
+    assert users(:kevin).can_administer_board?(boards(:writebook))
+
+    get edit_board_path(boards(:writebook))
+
+    assert_response :success
+    assert_select "button:not([disabled])", text: "Select all"
+    assert_select "button:not([disabled])", text: "Select none"
+  end
 end