feat(mail): create Calendar events from email

[imattau]

Description

Add a "create Calendar event from email" capability to Mail, with drag-and-drop as the primary desktop interaction and an "Add to Calendar" menu action as the accessible alternative.

Dropping a single Mail message onto the Calendar drawer's grid opens the standard event editor with:

• the email subject as the event title
• the drop position as the event start time (day/week) - or the dropped day's start time on month view
• a minimal From: <sender> description reference
• the user's normal default calendar and event defaults
• no automatic save

It reuses the existing Mail drag system, Calendar drawer iframe, drawer messaging protocol and Calendar event editor - no parallel drag or persistence path is introduced.

Implementation notes

• Three new typed drawer events in packages/shared/lib/drawer/interfaces.ts: CALENDAR_MAIL_DRAG_STATE, CALENDAR_MAIL_DROP and CALENDAR_CREATE_EVENT_FROM_MAIL.
• Mail layout state now tracks a single-message drag count and (only for single-message drags) the dragged ID; multi-message drags retain no ID in global state.
• The Calendar day/week/month grids become drop targets via useCalendarMailDrop; the slot date/time is computed with the existing getTargetMinutes/getNewTime helpers, then reported back to Mail via CALENDAR_MAIL_DROP.
• Mail resolves the dragged message and forwards only messageID, subject and sender metadata (never the body) via CALENDAR_CREATE_EVENT_FROM_MAIL.
• Calendar opens the existing editor through interactiveRef.createEventFromMail.
• "Add to Calendar" menu action opens the drawer (waiting for the iframe READY handshake on a cold open) and prefills subject/sender with default date/time.
• The Calendar drawer icon highlights as a drop target during a single-message drag and opens on hover.
• Interaction telemetry mail_calendar_create_event with a drag_drop / menu_action entry-point dimension (no message content).

Testing

New unit tests (which I verified pass via a local node --test harness, since I could not run the monorepo install):

• dateHelpers.spec.ts - drop-to-timestamp mapping for day, week and month grids (snapping, clamping, guards).
• createEventFromMessage.test.ts - only messageID/subject/sender forwarded (asserts no body), sender fallback, missing-subject normalisation.
• layoutSlice.spec.ts - drag count / dragged-ID transitions.
• drawer.spec.ts - the three new event types are recognised as drawer messages.

Note: I was unable to run yarn install in my environment (the @proton/* scope registry and vendor/* workspaces are only reachable from Proton's internal network), so the CI prettier / eslint / check-types gates have not been executed on my side. The changed files parse cleanly under TypeScript, but please run CI. Also, the new measurement group mail.web.calendar_create_event requires backend whitelisting or it will be silently dropped.

Out of scope

Month view maps a drop to the start of the dropped day (default-time event) rather than an exact time. Stable email backlinks, conversation-level events and date/time extraction are left for follow-ups.