fix(zen): always count cmdheight towards Zen bottom offset (#1402)

## Description

Zen window height should always account for `cmdheight` (not only if
`statusline` is enabled) and be computed dynamically when `cmdheight >
0` to avoid the `cmdline` obstructing the Zen window.

## Related Issue(s)

- Fixes https://github.com/folke/snacks.nvim/issues/1401
- Fixes https://github.com/folke/snacks.nvim/issues/1375

## Screenshots

Before:
![zen-mode-cmdline](https://github.com/user-attachments/assets/c03be4af-2368-4b04-8065-49ced875aa2d)

After:
![zen-mode-fixed-cmdline](https://github.com/user-attachments/assets/14dc72bb-7431-4d05-b0fd-b6a2367bdc6d)

lua/snacks/zen.lua

@@ -84,7 +84,7 @@ Snacks.util.set_hl({
 ---@param opts? {statusline: boolean, tabline: boolean}
 local function get_main(opts)
   opts = opts or {}
-  local bottom = opts.statusline and (vim.o.cmdheight + (vim.o.laststatus == 3 and 1 or 0)) or 0
+  local bottom = vim.o.cmdheight + (opts.statusline and vim.o.laststatus == 3 and 1 or 0)
   local top = opts.tabline
       and ((vim.o.showtabline == 2 or (vim.o.showtabline == 1 and #vim.api.nvim_list_tabpages() > 1)) and 1 or 0)
     or 0
@@ -127,7 +127,7 @@ function M.zen(opts)
   local show_indicator = false
 
   -- calculate window size
-  if win_opts.height == 0 and (opts.show.statusline or opts.show.tabline) then
+  if win_opts.height == 0 and (opts.show.statusline or opts.show.tabline or vim.o.cmdheight > 0) then
     local main = get_main(opts.show)
     win_opts.row = main.row
     win_opts.height = function()