Graph calendar sync: support multiple calendars

[mickenordin]

Currently sync_calendars_graph maps all events to the default calendar via GET /me/calendarView. This means:

• Non-default Graph calendars are discovered but their events aren't separated
• Unsubscribing from a non-default Graph calendar has no effect
• The subscription check only looks at the default calendar

Should be updated to:

1. Fetch events per-calendar using GET /me/calendars/{id}/events instead of the global calendarView
2. Use the same HashSet<String> unsubscribed filter pattern as JMAP/Google/CalDAV sync paths
3. Map events to their correct local calendar ID

See the TODO comment in src-tauri/src/commands/calendar.rs at sync_calendars_graph.