feat: race countdown support and language version selection (#346)

## Problem

Two gaps in the plugin: (1) contest pickers have no way to know whether
a contest supports countdown (race), so the UI can't surface that
affordance; (2) `:CP submit` hardcodes a single language ID per platform
with no way to choose C++ standard or override the platform ID.

## Solution

**Race countdown** (`4e709c8`): Add `supports_countdown` boolean to
`ContestListResult` and wire it through CSES/USACO scrapers, cache, race
module, and pickers.

**Language version selection** (`b90ac67`): Add `LANGUAGE_VERSIONS` and
`DEFAULT_VERSIONS` tables in `constants.lua`. Config gains `version` and
`submit_id` fields validated at setup time. `submit.lua` resolves the
effective config to a platform ID (priority: `submit_id` > `version` >
default). Helpdocs add `*cp-submit-language*` section. Tests cover
`LANGUAGE_IDS` completeness.

lua/cp/cache.lua

@@ -392,7 +392,8 @@ end
 
 ---@param platform string
 ---@param contests ContestSummary[]
-function M.set_contest_summaries(platform, contests)
+---@param opts? { supports_countdown?: boolean }
+function M.set_contest_summaries(platform, contests, opts)
   cache_data[platform] = cache_data[platform] or {}
   for _, contest in ipairs(contests) do
     cache_data[platform][contest.id] = cache_data[platform][contest.id] or {}
@@ -405,9 +406,22 @@ function M.set_contest_summaries(platform, contests)
     end
   end
 
+  if opts and opts.supports_countdown ~= nil then
+    cache_data[platform].supports_countdown = opts.supports_countdown
+  end
+
   M.save()
 end
 
+---@param platform string
+---@return boolean?
+function M.get_supports_countdown(platform)
+  if not cache_data[platform] then
+    return nil
+  end
+  return cache_data[platform].supports_countdown
+end
+
 ---@param platform string
 ---@param contest_id string
 ---@return integer?
@@ -418,6 +432,16 @@ function M.get_contest_start_time(platform, contest_id)
   return cache_data[platform][contest_id].start_time
 end
 
+---@param platform string
+---@param contest_id string
+---@return string?
+function M.get_contest_display_name(platform, contest_id)
+  if not cache_data[platform] or not cache_data[platform][contest_id] then
+    return nil
+  end
+  return cache_data[platform][contest_id].display_name
+end
+
 ---@param platform string
 ---@return table?
 function M.get_credentials(platform)