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/credentials.lua

@@ -11,6 +11,7 @@ local STATUS_MESSAGES = {
   installing_browser = 'Installing browser...',
 }
 
+---@param platform string?
 function M.login(platform)
   platform = platform or state.get_platform()
   if not platform then
@@ -68,6 +69,7 @@ function M.login(platform)
   end)
 end
 
+---@param platform string?
 function M.logout(platform)
   platform = platform or state.get_platform()
   if not platform then