BUG: Missing nil check on calendar event in renderCalendarInvite

[andrinoff]

Describe the bug

In tui/email_view.go:463-475, renderCalendarInvite() accesses event.Summary, event.Start, etc. without nil-checking the event parameter. If ParseICS() returns a successfully parsed calendar with nil event fields, this panics.

To reproduce

1. Receive an email with a minimal/malformed ICS attachment (e.g., VEVENT with no DTSTART)
2. Open the email
3. Panic on nil field access

Expected behavior

Check that event and its required fields are non-nil/non-zero before rendering.

[mavonx]

/assign