fix(utils): menu: ui.select menu has thick bottom border

Bug:    `ui.select` menu has improper thick bottom border when
        header/prompt is given and `opts.menu.win_configs.border` is
        configured as `{ '', '', '', '' }` (table with 4 elements)
Fix:    This is because we set the `border[2]` to ' ' (space).
        so that we have a thick upper border to show the header but
        the thick border is repeated as the bottom border when then
        the border table has only 4 elements, see `:h nvim_open_win`.
        To fix this issue, expand the border table to 8 elements
        and set the `border[2]` to space.

lua/dropbar/utils/menu.lua

@@ -183,9 +183,11 @@ function M.select(items, opts, on_choice)
         win_configs.border = border_none_with_prompt
         fzf_win_configs.border = 'none'
       elseif #border > 1 and border[2] == '' then
-        local border_cp = vim.deepcopy(border)
-        border_cp[2] = ' '
-        win_configs.border = border_cp
+        win_configs.border = vim.deepcopy(border)
+        if #win_configs.border == 4 then
+          vim.list_extend(win_configs.border, win_configs.border)
+        end
+        win_configs.border[2] = ' '
         -- use the original headerless border for fzf
         fzf_win_configs.border = border
       end