feat: silent background polling for dashboard and briefing views

HomeView: setInterval every 90s refreshes events, orphan tasks/notes,
and hero next-up task. Never touches loading ref, so the page content
stays stable — only the data silently swaps when the fetch completes.
Skips when document.hidden or initial load is still in progress.

BriefingView: setInterval every 60s refetches today's messages, but
only when viewing today's conversation and not currently streaming.
Compares message count and last message content before updating to
avoid unnecessary re-renders.

Both timers are cleared on unmount.

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

frontend/src/views/BriefingView.vue

@@ -189,9 +189,31 @@ function toMsg(m: BriefingMessage): Message {
   }
 }
 
+// ─── Background refresh (no-flicker) ─────────────────────────────────────────
+let _refreshTimer: ReturnType<typeof setInterval> | null = null
+
+async function _backgroundRefreshMessages() {
+  if (document.hidden || chatStore.streaming || !isToday.value || !todayConvId.value) return
+  try {
+    const today = await getBriefingToday()
+    if (!today) return
+    const fresh = today.messages
+    const last = fresh[fresh.length - 1]
+    const cur = messages.value[messages.value.length - 1]
+    if (fresh.length !== messages.value.length || last?.content !== cur?.content) {
+      messages.value = fresh
+    }
+  } catch { /* silent — don't disturb the UI on network hiccup */ }
+}
+
 onMounted(async () => {
   await checkSetup()
   if (!showWizard.value) await loadAll()
+  _refreshTimer = setInterval(_backgroundRefreshMessages, 60_000)
+})
+
+onUnmounted(() => {
+  if (_refreshTimer !== null) clearInterval(_refreshTimer)
 })
 </script>