feat: Add invite compressor and compressed header display

- Add InviteCompressor for terminal-friendly calendar invite summaries
- Add test fixtures for large group invite and cancellation emails
- Compress To/CC headers to single line with '... (+N more)' truncation
- Add 'h' keybinding to toggle between compressed and full headers
- EnvelopeHeader now shows first 2 recipients by default

tests/fixtures/test_mailbox/INBOX/cur/17051228-cancellation.test:2,S

@@ -0,0 +1,72 @@
+Content-Type: multipart/mixed; boundary="===============9876543210987654321=="
+MIME-Version: 1.0
+Message-ID: test-cancellation-001
+Subject: Canceled: Technical Refinement
+From: Marshall, Cody <cody.marshall@example.com>
+To: Ruttencutter, Chris <chris.ruttencutter@example.com>, Dake, Ryan <ryan.dake@example.com>,
+ Smith, James <james.smith@example.com>, Santana, Jonatas <jonatas.santana@example.com>
+Cc: Bendt, Timothy <timothy.bendt@example.com>
+Date: Fri, 19 Dec 2025 19:12:46 +0000
+Importance: high
+X-Priority: 1
+
+--===============9876543210987654321==
+Content-Type: text/plain; charset="us-ascii"
+MIME-Version: 1.0
+Content-Transfer-Encoding: 7bit
+
+The meeting has been cancelled.
+
+--===============9876543210987654321==
+Content-Type: text/calendar; charset="utf-8"; method=CANCEL
+Content-Transfer-Encoding: 7bit
+
+BEGIN:VCALENDAR
+METHOD:CANCEL
+PRODID:Microsoft Exchange Server 2010
+VERSION:2.0
+BEGIN:VTIMEZONE
+TZID:Central Standard Time
+BEGIN:STANDARD
+DTSTART:16010101T020000
+TZOFFSETFROM:-0500
+TZOFFSETTO:-0600
+RRULE:FREQ=YEARLY;INTERVAL=1;BYDAY=1SU;BYMONTH=11
+END:STANDARD
+BEGIN:DAYLIGHT
+DTSTART:16010101T020000
+TZOFFSETFROM:-0600
+TZOFFSETTO:-0500
+RRULE:FREQ=YEARLY;INTERVAL=1;BYDAY=2SU;BYMONTH=3
+END:DAYLIGHT
+END:VTIMEZONE
+BEGIN:VEVENT
+ORGANIZER;CN="Marshall, Cody":mailto:cody.marshall@example.com
+ATTENDEE;ROLE=REQ-PARTICIPANT;PARTSTAT=NEEDS-ACTION;RSVP=TRUE;CN="Ruttencutter, Chris":mailto:chris.ruttencutter@example.com
+ATTENDEE;ROLE=REQ-PARTICIPANT;PARTSTAT=NEEDS-ACTION;RSVP=TRUE;CN="Dake, Ryan":mailto:ryan.dake@example.com
+ATTENDEE;ROLE=REQ-PARTICIPANT;PARTSTAT=NEEDS-ACTION;RSVP=TRUE;CN="Smith, James":mailto:james.smith@example.com
+ATTENDEE;ROLE=REQ-PARTICIPANT;PARTSTAT=NEEDS-ACTION;RSVP=TRUE;CN="Santana, Jonatas":mailto:jonatas.santana@example.com
+ATTENDEE;ROLE=OPT-PARTICIPANT;PARTSTAT=NEEDS-ACTION;RSVP=TRUE;CN="Bendt, Timothy":mailto:timothy.bendt@example.com
+UID:040000008200E00074C5B7101A82E00800000000043F526712345678901000000000000000
+ 10000000308993960B03FD4C968B934EDD662857
+RECURRENCE-ID;TZID=Central Standard Time:20251224T133000
+SUMMARY;LANGUAGE=en-US:Canceled: Technical Refinement
+DTSTART;TZID=Central Standard Time:20251224T133000
+DTEND;TZID=Central Standard Time:20251224T140000
+CLASS:PUBLIC
+PRIORITY:1
+DTSTAMP:20251219T191240Z
+TRANSP:TRANSPARENT
+STATUS:CANCELLED
+SEQUENCE:84
+LOCATION;LANGUAGE=en-US:Microsoft Teams Meeting
+X-MICROSOFT-CDO-APPT-SEQUENCE:84
+X-MICROSOFT-CDO-BUSYSTATUS:FREE
+X-MICROSOFT-CDO-INTENDEDSTATUS:FREE
+X-MICROSOFT-CDO-ALLDAYEVENT:FALSE
+X-MICROSOFT-CDO-IMPORTANCE:2
+X-MICROSOFT-CDO-INSTTYPE:3
+END:VEVENT
+END:VCALENDAR
+
+--===============9876543210987654321==--