- Create CalendarInvitePanel widget showing event summary, time, location,
organizer, and attendees with accept/decline/tentative buttons
- Add is_calendar_email() to notification_detector for detecting invite emails
- Add get_raw_message() to himalaya client for exporting full MIME content
- Refactor calendar_parser.py with proper icalendar parsing (METHOD at
VCALENDAR level, not VEVENT)
- Integrate calendar panel into ContentContainer.display_content flow
- Update tests for new calendar parsing API
- Minor: fix today's header style in calendar WeekGrid
