feat(atcoder): extract submit helpers; add live status notifications (#294)

## Problem

`_submit_sync` was a 170-line nested closure with `_solve_turnstile` and
the browser-install block further nested inside it. Status events went
to
stderr, which `run_scraper()` silently discards, leaving the user with a
10–30s silent hang after credential entry. The NDJSON spawn path also
lacked stdin support, so submit had no streaming path at all.

## Solution

Extract `_TURNSTILE_JS`, `_solve_turnstile`, `_ensure_browser`, and
`_submit_headless` to module level in `atcoder.py`; status events
(`installing_browser`, `checking_login`, `logging_in`, `submitting`) now
print to stdout as NDJSON. Add stdin pipe support to the NDJSON spawn
path in `scraper.lua` and switch `M.submit` to streaming with an
`on_status` callback. Wire `on_status` in `submit.lua` to fire
`vim.notify` for each phase transition.

pyproject.toml

@@ -7,6 +7,7 @@ requires-python = ">=3.11"
 dependencies = [
     "backoff>=2.2.1",
     "beautifulsoup4>=4.13.5",
+    "scrapling[fetchers]>=0.4",
     "curl-cffi>=0.13.0",
     "httpx>=0.28.1",
     "ndjson>=0.3.1",