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/app.py

@@ -19,6 +19,7 @@ from textual.reactive import reactive
 from src.calendar.backend import CalendarBackend, Event
 from src.calendar.widgets.WeekGrid import WeekGrid
 from src.calendar.widgets.MonthCalendar import MonthCalendar
+from src.calendar.widgets.InvitesPanel import InvitesPanel, CalendarInvite
 from src.calendar.widgets.AddEventForm import EventFormData
 from src.utils.shared_config import get_theme_name
 
@@ -72,6 +73,13 @@ class CalendarApp(App):
     #sidebar-calendar {
         height: auto;
     }
+    
+    #sidebar-invites {
+        height: auto;
+        margin-top: 1;
+        border-top: solid $surface-darken-1;
+        padding-top: 1;
+    }
 
     #week-grid {
         height: 1fr;
@@ -120,6 +128,7 @@ class CalendarApp(App):
         Binding("g", "goto_today", "Today", show=True),
         Binding("w", "toggle_weekends", "Weekends", show=True),
         Binding("s", "toggle_sidebar", "Sidebar", show=True),
+        Binding("i", "focus_invites", "Invites", show=True),
         Binding("r", "refresh", "Refresh", show=True),
         Binding("enter", "view_event", "View", show=True),
         Binding("a", "add_event", "Add", show=True),
@@ -132,9 +141,11 @@ class CalendarApp(App):
 
     # Instance attributes
     backend: Optional[CalendarBackend]
+    _invites: list[CalendarInvite]
 
     def __init__(self, backend: Optional[CalendarBackend] = None):
         super().__init__()
+        self._invites = []
 
         if backend:
             self.backend = backend
@@ -150,6 +161,7 @@ class CalendarApp(App):
         with Horizontal(id="main-content"):
             with Vertical(id="sidebar"):
                 yield MonthCalendar(id="sidebar-calendar")
+                yield InvitesPanel(id="sidebar-invites")
             yield WeekGrid(id="week-grid")
         yield Static(id="event-detail", classes="hidden")
         yield CalendarStatusBar(id="status-bar")
@@ -165,10 +177,75 @@ class CalendarApp(App):
         # Sync sidebar calendar with current week
         self._sync_sidebar_calendar()
 
+        # Load invites in background
+        self.run_worker(self._load_invites_async(), exclusive=True)
+
         # Update status bar and title
         self._update_status()
         self._update_title()
 
+    async def _load_invites_async(self) -> None:
+        """Load pending calendar invites from Microsoft Graph."""
+        try:
+            from src.services.microsoft_graph.auth import get_access_token
+            from src.services.microsoft_graph.calendar import fetch_pending_invites
+            from dateutil import parser as date_parser
+
+            # Get auth token
+            scopes = ["https://graph.microsoft.com/Calendars.Read"]
+            _, headers = get_access_token(scopes)
+
+            # Fetch invites
+            raw_invites = await fetch_pending_invites(headers, days_forward=30)
+
+            # Convert to CalendarInvite objects
+            invites = []
+            for inv in raw_invites:
+                try:
+                    start_str = inv.get("start", {}).get("dateTime", "")
+                    end_str = inv.get("end", {}).get("dateTime", "")
+                    start_dt = (
+                        date_parser.parse(start_str) if start_str else datetime.now()
+                    )
+                    end_dt = date_parser.parse(end_str) if end_str else start_dt
+
+                    organizer_data = inv.get("organizer", {}).get("emailAddress", {})
+                    organizer_name = organizer_data.get(
+                        "name", organizer_data.get("address", "Unknown")
+                    )
+
+                    invite = CalendarInvite(
+                        id=inv.get("id", ""),
+                        subject=inv.get("subject", "No Subject"),
+                        organizer=organizer_name,
+                        start=start_dt,
+                        end=end_dt,
+                        location=inv.get("location", {}).get("displayName"),
+                        is_all_day=inv.get("isAllDay", False),
+                        response_status=inv.get("responseStatus", {}).get(
+                            "response", "notResponded"
+                        ),
+                    )
+                    invites.append(invite)
+                except Exception as e:
+                    logger.warning(f"Failed to parse invite: {e}")
+
+            # Update the panel
+            self._invites = invites
+            self.call_from_thread(self._update_invites_panel)
+
+        except Exception as e:
+            logger.warning(f"Failed to load invites: {e}")
+            # Silently fail - invites are optional
+
+    def _update_invites_panel(self) -> None:
+        """Update the invites panel with loaded invites."""
+        try:
+            panel = self.query_one("#sidebar-invites", InvitesPanel)
+            panel.set_invites(self._invites)
+        except Exception:
+            pass
+
     def _sync_sidebar_calendar(self) -> None:
         """Sync the sidebar calendar with the main week grid."""
         try:
@@ -443,6 +520,7 @@ Keybindings:
   g       - Go to today
   w       - Toggle weekends (5/7 days)
   s       - Toggle sidebar
+  i       - Focus invites panel
   Enter   - View event details
   a       - Add new event
   r       - Refresh
@@ -450,6 +528,16 @@ Keybindings:
 """
         self.notify(help_text.strip(), timeout=10)
 
+    def action_focus_invites(self) -> None:
+        """Focus on the invites panel and show invite count."""
+        if not self.show_sidebar:
+            self.action_toggle_sidebar()
+
+        if self._invites:
+            self.notify(f"You have {len(self._invites)} pending invite(s)")
+        else:
+            self.notify("No pending invites")
+
 
 def run_app(backend: Optional[CalendarBackend] = None) -> None:
     """Run the Calendar TUI application."""