fix: KnowledgeView infinite scroll root + calendar event refresh

- KnowledgeView: IntersectionObserver was watching the viewport instead
  of the card-grid scroll container, causing infinite scroll to stop
  loading after only ~29 items. Pass card-grid element as `root`.
- CalendarView: listen for 'fable:calendar-changed' custom event and
  call refetchEvents() so tool-created events appear without navigation.
- ChatPanel: dispatch 'fable:calendar-changed' when create_event,
  update_event, or delete_event tool calls succeed.
- tools.py: normalize naive datetimes to UTC before storing events so
  timezone comparisons in list_events queries are always consistent.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/fabledassistant/services/tools.py

@@ -3,7 +3,7 @@
 import asyncio
 import logging
 import re
-from datetime import date, datetime
+from datetime import date, datetime, timezone
 from difflib import SequenceMatcher
 
 from fabledassistant.services.caldav import (
@@ -1409,6 +1409,8 @@ async def execute_tool(
                 start_str = f"{start_str}T00:00:00"
             try:
                 start_dt = datetime.fromisoformat(start_str)
+                if start_dt.tzinfo is None:
+                    start_dt = start_dt.replace(tzinfo=timezone.utc)
             except (ValueError, TypeError):
                 return {"success": False, "error": f"Invalid start datetime: {start_str!r}"}
             end_dt = None
@@ -1417,6 +1419,8 @@ async def execute_tool(
                     end_str = f"{end_str}T00:00:00"
                 try:
                     end_dt = datetime.fromisoformat(end_str)
+                    if end_dt.tzinfo is None:
+                        end_dt = end_dt.replace(tzinfo=timezone.utc)
                 except (ValueError, TypeError):
                     return {"success": False, "error": f"Invalid end datetime: {end_str!r}"}
             project_id = None