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/runner/run_render.lua

@@ -376,6 +376,7 @@ function M.get_highlight_groups()
   }
 end
 
+---@return nil
 function M.setup_highlights()
   local groups = M.get_highlight_groups()
   for name, opts in pairs(groups) do