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>

frontend/src/views/CalendarView.vue

@@ -115,8 +115,18 @@ function onDocClick(e: MouseEvent) {
   }
 }
 
-onMounted(() => document.addEventListener("mousedown", onDocClick));
-onUnmounted(() => document.removeEventListener("mousedown", onDocClick));
+function onCalendarChanged() {
+  calendarRef.value?.getApi().refetchEvents();
+}
+
+onMounted(() => {
+  document.addEventListener("mousedown", onDocClick);
+  document.addEventListener("fable:calendar-changed", onCalendarChanged);
+});
+onUnmounted(() => {
+  document.removeEventListener("mousedown", onDocClick);
+  document.removeEventListener("fable:calendar-changed", onCalendarChanged);
+});
 
 // ── Calendar callbacks ─────────────────────────────────────────────────────
 async function loadEvents(