Fix a bug where magic mode and per-request UA overrides would change
the User-Agent header without updating the sec-ch-ua (browser hint)
header to match. Anti-bot systems like Akamai detect this mismatch
as a bot signal.

Changes:
- Regenerate browser_hint via UAGen.generate_client_hints() whenever
  the UA is changed at crawl time (magic mode or explicit override)
- Re-apply updated headers to the page via set_extra_http_headers()
- Skip per-crawl UA override for persistent contexts where the UA is
  locked at launch time by Playwright's protocol layer
- Move --disable-gpu flags behind enable_stealth check so WebGL works
  via SwiftShader when stealth mode is active (missing WebGL is a
  detectable headless signal)
- Clean up old test scripts, add clean anti-bot test