Microsoft / Outlook calendar provider

[Phoenix-]

Support Microsoft 365 / Outlook.com calendars alongside Google.

Scope

• New provider abstraction (current code is Google-only; refactor to interface).
• Microsoft Graph API integration: events + recurring + cancellations.
• MSAL auth (separate OAuth dance from Google).
• Account management: support both providers simultaneously, multiple accounts each.
• Calendar list UI updated to show provider per calendar.

Out of scope

• Microsoft Tasks / To Do (separate issue if wanted).
• Exchange on-prem (Graph only).

Notes

• Big issue. Worth breaking into sub-issues once the abstraction shape is clearer.
• Should land after Google flow is stable in RW mode — but we're shipping read-only first, so this can land before RW.