Add calendar invites panel to Calendar TUI sidebar

- 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

src/calendar/widgets/__init__.py

@@ -3,5 +3,13 @@
 from .WeekGrid import WeekGrid
 from .AddEventForm import AddEventForm, EventFormData
 from .MonthCalendar import MonthCalendar
+from .InvitesPanel import InvitesPanel, CalendarInvite
 
-__all__ = ["WeekGrid", "AddEventForm", "EventFormData", "MonthCalendar"]
+__all__ = [
+    "WeekGrid",
+    "AddEventForm",
+    "EventFormData",
+    "MonthCalendar",
+    "InvitesPanel",
+    "CalendarInvite",
+]