feat(codechef): implement full CodeChef support (#354)

## Problem

CodeChef had no working login, submit, or contest list. The browser
selectors were wrong, the contest list was missing present/past
contests,
and problem/contest URLs were unset.

## Solution

Fix login and submit selectors for the Drupal-based site. Paginate
`/api/list/contests/past` to collect all 228 Starters, then expand each
parent contest into individual division entries (e.g. `START228 (Div.
4)`).
Add language IDs, correct `url`/`contest_url`/`standings_url` in
metadata,
and make `:CP <platform>` open the contest picker directly.

lua/cp/stress.lua

@@ -58,6 +58,8 @@ local function build_run_cmd(file)
   return './' .. file
 end
 
+---@param generator_cmd? string
+---@param brute_cmd? string
 function M.toggle(generator_cmd, brute_cmd)
   if state.get_active_panel() == 'stress' then
     if state.stress_buf and vim.api.nvim_buf_is_valid(state.stress_buf) then
@@ -239,6 +241,7 @@ function M.toggle(generator_cmd, brute_cmd)
   end)
 end
 
+---@return nil
 function M.cancel()
   if state.stress_buf and vim.api.nvim_buf_is_valid(state.stress_buf) then
     local job = vim.b[state.stress_buf].terminal_job_id