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/services/microsoft_graph/calendar.py

@@ -468,3 +468,76 @@ async def sync_local_calendar_changes(
         )
 
     return created_count, deleted_count
+
+
+async def fetch_pending_invites(headers, days_forward=30):
+    """
+    Fetch calendar invites that need a response (pending/tentative).
+
+    Args:
+        headers (dict): Headers including authentication.
+        days_forward (int): Number of days to look forward.
+
+    Returns:
+        list: List of invite dictionaries with response status info.
+    """
+    start_date = datetime.now()
+    end_date = start_date + timedelta(days=days_forward)
+
+    start_date_str = start_date.strftime("%Y-%m-%dT00:00:00Z")
+    end_date_str = end_date.strftime("%Y-%m-%dT23:59:59Z")
+
+    # Fetch events with response status
+    calendar_url = (
+        f"https://graph.microsoft.com/v1.0/me/calendarView?"
+        f"startDateTime={start_date_str}&endDateTime={end_date_str}&"
+        f"$select=id,subject,organizer,start,end,location,isAllDay,responseStatus,isCancelled&"
+        f"$filter=responseStatus/response eq 'notResponded' or responseStatus/response eq 'tentativelyAccepted'&"
+        f"$orderby=start/dateTime"
+    )
+
+    invites = []
+
+    try:
+        response_data = await fetch_with_aiohttp(calendar_url, headers)
+        invites.extend(response_data.get("value", []))
+
+        # Handle pagination
+        next_link = response_data.get("@odata.nextLink")
+        while next_link:
+            response_data = await fetch_with_aiohttp(next_link, headers)
+            invites.extend(response_data.get("value", []))
+            next_link = response_data.get("@odata.nextLink")
+
+    except Exception as e:
+        print(f"Error fetching pending invites: {e}")
+
+    return invites
+
+
+async def respond_to_invite(headers, event_id, response):
+    """
+    Respond to a calendar invite.
+
+    Args:
+        headers (dict): Authentication headers
+        event_id (str): The ID of the event to respond to
+        response (str): Response type - 'accept', 'tentativelyAccept', or 'decline'
+
+    Returns:
+        bool: True if response was successful
+    """
+    valid_responses = ["accept", "tentativelyAccept", "decline"]
+    if response not in valid_responses:
+        print(f"Invalid response type: {response}. Must be one of {valid_responses}")
+        return False
+
+    try:
+        response_url = (
+            f"https://graph.microsoft.com/v1.0/me/events/{event_id}/{response}"
+        )
+        status = await post_with_aiohttp(response_url, headers, {})
+        return status in (200, 202)
+    except Exception as e:
+        print(f"Error responding to invite: {e}")
+        return False