fix(events): audit pass — 7 correctness fixes across the events system

Backend:
- tools.py: apply UTC normalization to update_event datetime fields
  (matched create_event which already did this)
- events.py service: allow end_dt/recurrence/project_id to be cleared
  via update_event by permitting None for nullable fields
- events.py service: find_events_by_query now returns upcoming events
  first, falling back to past — prevents AI tools from mutating stale
  past events when a future match exists
- events.py service: list_events now uses overlap logic (start <= to
  AND end >= from) so multi-day events spanning the query boundary
  are included; previously only start_dt was checked

Frontend:
- ToolCallCard: fire fable:calendar-changed on created/updated/deleted
  so CalendarView refetches without requiring a manual page refresh
- KnowledgeView: replace raw apiGet('/api/events') with listEvents()
  client function; also fix today bar which was reading .events off a
  flat array (always empty) — now correctly receives EventEntry[]
- HomeView: use full ISO strings for event date range instead of naive
  UTC-midnight strings; deduplicate inline date math via _dateRange()

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

src/fabledassistant/services/tools.py

@@ -1506,7 +1506,10 @@ async def execute_tool(
                 val = arguments.get(key)
                 if val:
                     try:
-                        fields[dt_field] = datetime.fromisoformat(val)
+                        dt = datetime.fromisoformat(val.replace("Z", "+00:00"))
+                        if dt.tzinfo is None:
+                            dt = dt.replace(tzinfo=timezone.utc)
+                        fields[dt_field] = dt
                     except (ValueError, TypeError):
                         return {"success": False, "error": f"Invalid datetime for {key}: {val!r}"}
             updated = await events_update_event(