fix: email persistence, HH-User-Agent header, friendly modal UI, docs

- api_client: renamed UA from HHAnalyticsBot→HHAnalytics (DDoS Guard trigger),
  added required HH-User-Agent header (HH.ru requirement since 2024)
- main.py: init user_settings from .env at startup; pass email to both
  collectors via HHAPIClient(email=); regex=→pattern= (FastAPI deprecation)
- index.html: friendly email modal redesign — welcoming tone, blue info
  block, 'Позже' button instead of amber warnings
- README.md: complete rewrite — correct startup command, all 20+ API
  endpoints, 403/DDoS Guard troubleshooting, version bumped to 1.2.0

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: watchmanheavy