Send METHOD:CANCEL for self-sent calendar cancellations

[mickenordin]

Problem

The event action labelled “Send Cancellation” calls the ordinary
invitation path before deleting the event. That path generates
METHOD:REQUEST, so attendees of a self-sent JMAP or generic
IMAP/CalDAV meeting can receive another invitation instead of a
cancellation.

Gmail and Microsoft Graph invitation delivery remains
provider-owned and must not gain a duplicate manual message.

Scope

• Generate a standards-compliant METHOD:CANCEL for providers
  where Chithi self-sends meeting messages.
• Preserve the event UID, organizer, attendees, recurrence data,
  sequence, and available timestamps from the stored event.
• Send the cancellation successfully before deleting local state.
• Preserve provider-delegated cancellation for Gmail and Graph.

Acceptance criteria

• “Send Cancellation” never emits METHOD:REQUEST.
• JMAP and generic IMAP/CalDAV attendees receive one CANCEL with
  the original event identity.
• A failed self-send does not silently delete the event.
• Gmail and Graph do not receive duplicate manual invitation mail.
• Focused generator and command-path tests cover both delivery
  ownership modes.

Out of scope

• Sync-time processing of unopened iTIP messages.
• A generalized iTIP revision or idempotency engine.
• Durable calendar-message delivery, which remains tracked by
  Persist calendar invite and RSVP delivery #261.

Evidence

• src/components/calendar/EventDetail.vue:166-190
• src-tauri/src/commands/calendar.rs:1835-1852
• src-tauri/src/calendar/ical.rs:260-341
• docs/adr/0038-calendar-invite-provider-delegation.md