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/config.lua

@@ -531,10 +531,12 @@ end
 
 local current_config = nil
 
+---@param config cp.Config
 function M.set_current_config(config)
   current_config = config
 end
 
+---@return cp.Config
 function M.get_config()
   return current_config or M.defaults
 end