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/components/ChatPanel.vue

@@ -76,6 +76,19 @@ function scrollToBottom() {
 watch(() => store.streamingContent, () => scrollToBottom())
 watch(() => store.currentConversation?.messages.length, () => scrollToBottom())
 
+// Notify CalendarView when an event is created/updated/deleted via tool
+const _calendarToolNames = new Set(['create_event', 'update_event', 'delete_event'])
+const _seenCalendarToolIdx = new Set<number>()
+watch(() => store.streamingToolCalls, (calls) => {
+  calls.forEach((tc, i) => {
+    if (!_seenCalendarToolIdx.has(i) && _calendarToolNames.has(tc.function) && tc.status === 'success') {
+      _seenCalendarToolIdx.add(i)
+      document.dispatchEvent(new CustomEvent('fable:calendar-changed'))
+    }
+  })
+}, { deep: true })
+watch(() => store.streaming, (s) => { if (!s) _seenCalendarToolIdx.clear() })
+
 // ── RAG scope chip (full, non-briefing, non-workspace) ────────────────────────
 const projects = ref<{ id: number; title: string }[]>([])
 const scopeDropdownOpen = ref(false)