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/KnowledgeView.vue

@@ -333,11 +333,12 @@ function formatDate(iso: string): string {
 // ─── Infinite scroll ──────────────────────────────────────────────────────────
 
 const sentinelEl = ref<HTMLElement | null>(null);
+const cardGridEl = ref<HTMLElement | null>(null);
 let scrollObserver: IntersectionObserver | null = null;
 
 function setupScrollObserver() {
   if (scrollObserver) scrollObserver.disconnect();
-  if (!sentinelEl.value) return;
+  if (!sentinelEl.value || !cardGridEl.value) return;
   scrollObserver = new IntersectionObserver(
     (entries) => {
       if (entries[0].isIntersecting && !loading.value && items.value.length < total.value) {
@@ -345,7 +346,7 @@ function setupScrollObserver() {
         fetchItems();
       }
     },
-    { rootMargin: "200px" }
+    { root: cardGridEl.value, rootMargin: "200px" }
   );
   scrollObserver.observe(sentinelEl.value);
 }
@@ -368,7 +369,7 @@ onUnmounted(() => {
 });
 
 watchEffect(() => {
-  if (sentinelEl.value) setupScrollObserver();
+  if (sentinelEl.value && cardGridEl.value) setupScrollObserver();
 });
 </script>
 
@@ -471,7 +472,7 @@ watchEffect(() => {
         </div>
 
         <!-- Card grid -->
-        <div v-else class="card-grid">
+        <div v-else ref="cardGridEl" class="card-grid">
           <div
             v-for="item in items"
             :key="item.id"