Fix meeting alert catch-up window and skip redundant block switch

- Extend start-time trigger window from 15s to 5 min so opening the
  dashboard late still marks the meeting as started
- Only play chime and show TV overlay when within 30s of start time
- Skip switchBlock call when the session is already on that block

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

frontend/src/views/DashboardView.vue

@@ -283,6 +283,7 @@ const scheduleStore = useScheduleStore()
 const activeChild = computed(() => childrenStore.activeChild)
 const meetingAlerts = useMeetingAlerts((blockId) => {
   if (!scheduleStore.session) return
+  if (scheduleStore.session.current_block_id === blockId) return
   scheduleStore.switchBlock(scheduleStore.session.id, blockId)
 })