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/commands/picker.lua

@@ -5,8 +5,9 @@ local logger = require('cp.log')
 
 --- Dispatch `:CP pick` to appropriate picker
 ---@param language? string
+---@param platform? string
 ---@return nil
-function M.handle_pick_action(language)
+function M.handle_pick_action(language, platform)
   local config = config_module.get_config()
 
   if not (config.ui and config.ui.picker) then
@@ -54,7 +55,7 @@ function M.handle_pick_action(language)
     picker = fzf_picker
   end
 
-  picker.pick(language)
+  picker.pick(language, platform)
 end
 
 return M