feat(cses): implement submit via REST API (#299)

## Problem

CSES submit was a stub returning "not yet implemented".

## Solution

Authenticate via web login + API token bridge (POST `/login` form, then
POST `/api/login` and confirm the auth page), submit source to
`/api/courses/problemset/submissions` with base64-encoded content, and
poll for verdict. Uses the same username/password credential model as
AtCoder — no browser dependencies needed. Tested end-to-end with a real
CSES account (verdict: `ACCEPTED`).

Also updates `scraper.lua` to pass the full ndjson event object to
`on_status` and handle `credentials` events for future platform use.

lua/cp/scraper.lua

@@ -298,9 +298,12 @@ function M.submit(
     stdin = source_code,
     env_extra = { CP_CREDENTIALS = vim.json.encode(credentials) },
     on_event = function(ev)
+      if ev.credentials ~= nil then
+        require('cp.cache').set_credentials(platform, ev.credentials)
+      end
       if ev.status ~= nil then
         if type(on_status) == 'function' then
-          on_status(ev.status)
+          on_status(ev)
         end
       elseif ev.success ~= nil then
         done = true