Problem: `:CP <platform> login` blindly caches credentials without
validating them against the platform.
Solution: add `LoginResult` to `models.py`, abstract `login()` method
and `_login_error` helper to `BaseScraper`, and wire up the `"login"`
CLI dispatch in `_run_cli_async`.
Problem: problem pages contain floating-point precision requirements and
contest start timestamps that were not being extracted or stored. The
submit workflow also needed a foundation in the scraper layer.
Solution: add extract_precision() to base.py and propagate through all
scrapers into cache. Add start_time to ContestSummary and extract it
from AtCoder and Codeforces. Add SubmitResult model, abstract submit()
method, submit CLI case with get_language_id() resolution, stdin/env_extra
support in run_scraper, and a full AtCoder submit implementation; stub
the remaining platforms.
