Add a lightweight preflight check for cookies/proxy before starting a crawl

I've been using MediaCrawler with saved cookies for a few platforms, and the most common failure mode is not obvious until the crawl has already started: the session expired, the proxy is blocked, or the cookie file is for the wrong account/platform.

A small diagnostic command would save a lot of trial and error. For example, python main.py --check-session --platform xhs could load the current login state, make one low-impact authenticated request, and print whether the session is usable. It could also mention the likely cause if it fails, such as expired cookie, proxy error, or unexpected response.

This doesn't need to auto-fix login or refresh tokens. Even a simple pass/fail status for each configured platform would make setup and scheduled runs much less painful, especially when multiple platforms are involved.

Start at main.py and trace how the --platform option selects a platform and loads its current login state. Review the existing authenticated requests and proxy handling before defining a low-impact check for each configured platform. Done means --check-session reports a clear pass or failure reason without starting a crawl or changing login state.