feat: Add CalendarInvitePanel to display invite details in mail app

- 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

tests/test_calendar_parsing.py

@@ -1,7 +1,14 @@
 """Unit tests for calendar email detection and ICS parsing."""
 
 import pytest
-from src.mail.utils import calendar_parser
+from src.mail.utils.calendar_parser import (
+    parse_ics_content,
+    parse_calendar_from_raw_message,
+    extract_ics_from_mime,
+    is_cancelled_event,
+    is_event_request,
+    ParsedCalendarEvent,
+)
 from src.mail.notification_detector import is_calendar_email
 
 
@@ -44,58 +51,91 @@ class TestICSParsing:
     """Test ICS file parsing."""
 
     def test_parse_cancellation_ics(self):
-        """Test parsing of cancellation ICS from test fixture."""
-        import base64
-        from pathlib import Path
+        """Test parsing of cancellation ICS."""
+        ics_content = """BEGIN:VCALENDAR
+VERSION:2.0
+PRODID:-//Test//Test//EN
+METHOD:CANCEL
+BEGIN:VEVENT
+UID:test-uid-001
+SUMMARY:Technical Refinement Meeting
+DTSTART:20251230T140000Z
+DTEND:20251230T150000Z
+STATUS:CANCELLED
+ORGANIZER;CN=Test Organizer:mailto:organizer@example.com
+ATTENDEE;CN=Test Attendee:mailto:attendee@example.com
+END:VEVENT
+END:VCALENDAR"""
 
-        fixture_path = Path(
-            "tests/fixtures/test_mailbox/INBOX/cur/17051226-calendar-invite-001.test:2"
-        )
-        if not fixture_path.exists():
-            pytest.skip("Test fixture file not found")
-            return
-
-        with open(fixture_path, "r") as f:
-            content = f.read()
-
-        event = parse_calendar_part(content)
+        event = parse_ics_content(ics_content)
         assert event is not None
         assert is_cancelled_event(event) is True
         assert event.method == "CANCEL"
         assert event.summary == "Technical Refinement Meeting"
 
     def test_parse_invite_ics(self):
-        """Test parsing of invite ICS from test fixture."""
-        import base64
-        from pathlib import Path
+        """Test parsing of invite/request ICS."""
+        ics_content = """BEGIN:VCALENDAR
+VERSION:2.0
+PRODID:-//Test//Test//EN
+METHOD:REQUEST
+BEGIN:VEVENT
+UID:test-uid-002
+SUMMARY:Team Standup
+DTSTART:20251230T100000Z
+DTEND:20251230T103000Z
+STATUS:CONFIRMED
+ORGANIZER;CN=Test Organizer:mailto:organizer@example.com
+ATTENDEE;CN=Test Attendee:mailto:attendee@example.com
+LOCATION:Conference Room A
+END:VEVENT
+END:VCALENDAR"""
 
-        fixture_path = Path(
-            "tests/fixtures/test_mailbox/INBOX/cur/17051226-calendar-invite-001.test:2"
-        )
-        if not fixture_path.exists():
-            pytest.skip("Test fixture file not found")
-            return
-
-        with open(fixture_path, "r") as f:
-            content = f.read()
-
-        event = parse_calendar_part(content)
+        event = parse_ics_content(ics_content)
         assert event is not None
         assert is_event_request(event) is True
         assert event.method == "REQUEST"
-        assert event.summary == "Technical Refinement Meeting"
+        assert event.summary == "Team Standup"
+        assert event.location == "Conference Room A"
 
     def test_invalid_ics(self):
         """Test parsing of invalid ICS content."""
-        event = parse_calendar_part("invalid ics content")
-
+        event = parse_ics_content("invalid ics content")
         assert event is None  # Should return None for invalid ICS
 
-    def test_base64_decoding(self):
-        """Test base64 decoding of ICS attachment."""
-        # Test that we can decode base64
-        encoded = "BASE64ENCODED_I_TEST"
-        import base64
+    def test_extract_ics_from_mime(self):
+        """Test extraction of ICS from raw MIME message."""
+        raw_message = """From: organizer@example.com
+To: attendee@example.com
+Subject: Meeting Invite
+Content-Type: multipart/mixed; boundary="boundary123"
 
-        decoded = base64.b64decode(encoded)
-        assert decoded == encoded
+--boundary123
+Content-Type: text/plain
+
+You have been invited to a meeting.
+
+--boundary123
+Content-Type: text/calendar
+
+BEGIN:VCALENDAR
+VERSION:2.0
+METHOD:REQUEST
+BEGIN:VEVENT
+UID:mime-test-001
+SUMMARY:MIME Test Meeting
+DTSTART:20251230T140000Z
+DTEND:20251230T150000Z
+END:VEVENT
+END:VCALENDAR
+--boundary123--
+"""
+        ics = extract_ics_from_mime(raw_message)
+        assert ics is not None
+        assert "BEGIN:VCALENDAR" in ics
+        assert "MIME Test Meeting" in ics
+
+        event = parse_ics_content(ics)
+        assert event is not None
+        assert event.summary == "MIME Test Meeting"
+        assert event.method == "REQUEST"