Moving toward external calendar support

[yujonglee]

Current Status

• We only support apple-calendar, powered by plugin/apple-calendar.
• Only macOS permissions are required. No network calls, no OAuth.
• We are still not 100% confident in its correctness.
  • (@goranmoomin is working on Apple Calendar Support Stabilize #2778)
• Unlike V0, after moving to V1, we now actively accept logins. So we can support all types of external integrations.
• Events are synced, not fetched on demand. This is by design.

What we're aiming to do

• Support google-calendar and outlook-calendar, alongside apple-calendar.
• Share as much code as possible.

What I'm proposing

Use nango.dev for OAuth

• Open source; we can use YC credits.
• @yujonglee is personally familiar with it.
• The most flexible solution on the market, as far as I know.
• Can be used for other types of integrations in the future.
• We have a Rust client implemented in crates/nango, and integration code in crates/api-nango.

Implementation details

• Nango has the concepts of proxy and sync.
• We will not use their "sync", at least for calendar integrations.
• Since we already support apple-calendar locally, which handles fetching/syncing on the desktop app side, maintaining the same pattern for external calendars makes more sense.
• Our Rust-based API server (see crates/api-nango and crates/api-calendar) should expose routes, and the client should use those, similar to how we currently do .listEvents( for apple-calendar.