fix: guard original_bufnr validity on borrowed restore

If the user deletes the original empty buffer (:bw!) between
hide and restore, nvim_win_set_buf would error. Now falls back
to closing the window if the original buffer is invalid.

lua/QQdock/init.lua

@@ -121,11 +121,15 @@ function M.open(cmd)
         vim.notify('QQdock: hide [' .. name .. ']', vim.log.levels.INFO)
       end
       if term.borrowed then
-        -- 占了主窗口 → 换回原始空 buffer
+        -- 占了主窗口 → 换回原始空 buffer（如果还在的话，否则关窗）
+        if term.original_bufnr and vim.api.nvim_buf_is_valid(term.original_bufnr) then
           vim.api.nvim_win_set_buf(term.winid, term.original_bufnr)
         else
           vim.api.nvim_win_close(term.winid, true)
         end
+      else
+        vim.api.nvim_win_close(term.winid, true)
+      end
       term.winid = nil
       term.borrowed = nil
       return