[Feature] Improve LLM prompt and add sender frequency tracking

[Feature] Improve LLM prompt and add sender frequency tracking

* Update default prompt to reduce false positives on legitimate emails
  - Explicitly recognize verification emails as legitimate
  - Require MULTIPLE red flags for phishing classification
  - Add guidance on known/frequent senders
* Add sender frequency detection in context
  - Classify senders as: new, occasional, known, frequent
  - Based on sender_counts from user context
  - Passed to LLM via context snippet
* Prompt instructs LLM to reduce phishing score for known senders
* Helps avoid false positives on transactional/verification emails