fix: knowledge infinite scroll + list_events timezone handling

KnowledgeView: sentinel was OUTSIDE the card-grid div, making
IntersectionObserver (root: cardGridEl) never fire since the target must
be a descendant of the root. Moved sentinel inside card-grid with
grid-column:1/-1 + order:9999 so it spans all columns and sits at the
bottom. Fixed backup check to compare against container bounds not viewport.

tools.py list_events: apply same UTC normalization as create_event (treat
naive datetimes as UTC, handle Z suffix). Update tool description to
explicitly request full-day UTC ranges so the LLM doesn't send local time
without offsets, which caused the recall query to miss UTC-stored events.

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

frontend/src/views/KnowledgeView.vue

@@ -83,13 +83,16 @@ async function fetchItems(reset = false) {
     loading.value = false;
   }
 
-  // If the sentinel is still in the viewport after appending, keep loading
+  // If the sentinel is still visible in the card-grid after appending, keep loading
   if (!reset && items.value.length < total.value) {
     await nextTick();
-    const rect = sentinelEl.value?.getBoundingClientRect();
-    if (rect && rect.top < window.innerHeight + 200) {
-      page.value++;
-      await fetchItems();
+    if (sentinelEl.value && cardGridEl.value) {
+      const containerBottom = cardGridEl.value.getBoundingClientRect().bottom;
+      const sentinelTop = sentinelEl.value.getBoundingClientRect().top;
+      if (sentinelTop < containerBottom + 200) {
+        page.value++;
+        await fetchItems();
+      }
     }
   }
 }
@@ -390,8 +393,13 @@ watchEffect(() => {
           <p v-else class="empty-hint">Start by creating a note, saving a person or place, or making a list.</p>
         </div>
 
-        <!-- Card grid -->
+        <!-- Card grid (sentinel lives inside so IntersectionObserver root works) -->
         <div v-else ref="cardGridEl" class="card-grid">
+          <!-- Infinite scroll sentinel — must be inside the scrolling root -->
+          <div ref="sentinelEl" class="scroll-sentinel">
+            <span v-if="loading && items.length > 0" class="sentinel-loading">Loading…</span>
+          </div>
+
           <div
             v-for="item in items"
             :key="item.id"
@@ -456,11 +464,6 @@ watchEffect(() => {
             </div>
           </div>
         </div>
-
-        <!-- Infinite scroll sentinel -->
-        <div ref="sentinelEl" class="scroll-sentinel">
-          <span v-if="loading && items.length > 0" class="sentinel-loading">Loading…</span>
-        </div>
       </div>
 
       <!-- Graph panel -->
@@ -888,11 +891,12 @@ watchEffect(() => {
 
 /* ── Infinite scroll sentinel ────────────────────────────── */
 .scroll-sentinel {
+  grid-column: 1 / -1; /* span all grid columns */
+  order: 9999;         /* push to end of grid items */
   height: 40px;
   display: flex;
   align-items: center;
   justify-content: center;
-  flex-shrink: 0;
 }
 .sentinel-loading {
   font-size: 0.8rem;