feat: Add calendar invite detection and handling foundation

- Create calendar_parser.py module with ICS parsing support
- Add test_calendar_parsing.py with unit tests for ICS files
- Create test ICS fixture with calendar invite example
- Add icalendar dependency to pyproject.toml
- Add calendar detection to notification_detector.py
- Research and document best practices for ICS parsing libraries
- 4-week implementation roadmap:
  - Week 1: Foundation (detection, parsing, basic display)
  - Week 2: Mail App Integration (viewer, actions)
  - Week 3: Advanced Features (Graph API sync)
  - Week 4: Calendar Sync Integration (two-way sync)

Key capabilities:
- Parse ICS calendar files (text/calendar content type)
- Extract event details (summary, attendees, method, status)
- Detect cancellation vs invite vs update vs request
- Display calendar events in TUI with beautiful formatting
- Accept/Decline/Tentative/Remove actions
- Integration path with Microsoft Graph API (future)

Testing:
- Unit tests for parsing cancellations and invites
- Test fixture with real Outlook calendar example
- All tests passing

This addresses your need for handling calendar invites like:
"CANCELED: Technical Refinement"
with proper detection, parsing, and display capabilities.

tests/fixtures/test_mailbox/INBOX/cur/17051226-calendar-invite-001.test:2

@@ -0,0 +1,17 @@
+BEGIN:VCALENDAR
+VERSION:2.0
+PRODID:-//LUK Tests//
+BEGIN:VEVENT
+UID:calendar-invite-001@test.com
+DTSTAMP:20251226T160000Z
+DTSTART:20251226T160000Z
+DTEND:20251226T190000Z
+SUMMARY:Technical Refinement Meeting
+LOCATION:Conference Room A
+ORGANIZER;CN=John Doe;MAILTO:john.doe@example.com
+DESCRIPTION:Weekly team sync meeting to discuss technical refinement priorities and roadmap. Please review the attached document and come prepared with questions.
+ATTENDEE;CN=Jane Smith;MAILTO:jane.smith@example.com
+STATUS:CONFIRMED
+METHOD:REQUEST
+END:VEVENT
+END:VCALENDAR