- Create InvitesPanel widget showing pending invites from Microsoft Graph
- Add fetch_pending_invites() and respond_to_invite() API functions
- Invites load asynchronously in background on app mount
- Display invite subject, date/time, and organizer
- Add 'i' keybinding to focus invites panel
- Style: tentative invites shown in warning color
- Add MonthCalendar widget as a collapsible sidebar (toggle with 's')
- Sidebar syncs with main week grid (week highlight, selected date)
- Click dates in sidebar to navigate week grid to that date
- Click month navigation arrows to change displayed month
- Add goto_date() method to WeekGrid for date navigation