Add timeout configuration option for API requests

[lzh-xdu]

Problem
When using leetcode.nvim with leetcode.cn, API requests sometimes timeout with the default 10s limit from
plenary.curl. This causes errors like:

'curl ...' was unable to complete in 10000 ms

This happens especially when fetching question details (by_title_slug query).

Current Behavior

• plenary.nvim has hardcoded 10s timeout
• leetcode.nvim doesn't pass timeout parameter to plenary.curl
• No user configuration exists to adjust this

Proposed Solution
Add a timeout option in leetcode.nvim config:

require("leetcode").setup({
  timeout = 30000,  -- ms, default 10000
})

Pass this to plenary.curl calls in lua/leetcode/api/utils.lua.

Workaround
Currently users must patch plenary.nvim/lua/plenary/curl.lua line 326 to increase timeout, but this gets
overwritten on Lazy sync.

Environment
- leetcode.cn (Chinese server, slower response)
- Network conditions vary