fix: correct date calculation in comprehensiveConversationSearch

The calculateTimeRange method was using setDate() which doesn't handle large
day offsets correctly. When subtracting 180 days from August 1st, it would
calculate -179 which JavaScript misinterprets, resulting in future dates.

Changed to use setTime() with millisecond calculation which properly handles
any number of days in the past.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: zackkatz