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/pickers/telescope.lua

@@ -64,7 +64,14 @@ local function contest_picker(opts, platform, refresh, language)
     :find()
 end
 
-function M.pick(language)
+---@param language? string
+---@param platform? string
+function M.pick(language, platform)
+  if platform then
+    contest_picker({}, platform, false, language)
+    return
+  end
+
   local opts = {}
   local platforms = picker_utils.get_platforms()