Add Meeting system subject and notification system

- Auto-create a locked "Meeting" subject for every user on registration
  and seed it for all existing users on startup
- Meeting subject cannot be deleted or renamed (is_system flag)
- 5-minute corner toast warning on Dashboard and TV with live countdown,
  dismiss button, and 1-minute re-notify if dismissed
- At start time: full-screen TV overlay with 30-second auto-dismiss,
  automatic pause of running block, switch to Meeting block, and
  auto-start of Meeting timer
- Web Audio API chimes: rising on warnings, falling at meeting start
- Update README with Meeting subject and notification system docs

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

frontend/src/composables/useMeetingAlerts.js

@@ -0,0 +1,132 @@
+import { ref, computed, onUnmounted } from 'vue'
+import { useScheduleStore } from '@/stores/schedule'
+
+function playChime(ascending = true) {
+  try {
+    const ctx = new (window.AudioContext || window.webkitAudioContext)()
+    const freqs = ascending ? [523, 659, 784] : [784, 659, 523]
+    freqs.forEach((freq, i) => {
+      const osc = ctx.createOscillator()
+      const gain = ctx.createGain()
+      osc.connect(gain)
+      gain.connect(ctx.destination)
+      osc.frequency.value = freq
+      gain.gain.setValueAtTime(0.3, ctx.currentTime + i * 0.18)
+      gain.gain.exponentialRampToValueAtTime(0.001, ctx.currentTime + i * 0.18 + 0.45)
+      osc.start(ctx.currentTime + i * 0.18)
+      osc.stop(ctx.currentTime + i * 0.18 + 0.5)
+    })
+  } catch (_) { /* audio unavailable */ }
+}
+
+export function useMeetingAlerts(onMeetingStart = null) {
+  const scheduleStore = useScheduleStore()
+  const now = ref(new Date())
+  const triggered = new Set()
+
+  // Active corner toasts for dashboard: [{ id, blockId, label }]
+  const dashboardAlerts = ref([])
+
+  // Full-screen TV alert: null | { blockId, label, autoDismissAt }
+  const tvAlert = ref(null)
+
+  const meetingBlocks = computed(() =>
+    (scheduleStore.blocks || []).filter(b => b.subject?.is_system)
+  )
+
+  function secsUntil(block) {
+    if (!block.time_start) return null
+    const [h, m, s] = block.time_start.split(':').map(Number)
+    const target = new Date(now.value)
+    target.setHours(h, m, s || 0, 0)
+    return Math.round((target - now.value) / 1000)
+  }
+
+  function blockLabel(block) {
+    return block.label || block.subject?.name || 'Meeting'
+  }
+
+  // Live countdown string for a dashboard alert
+  function alertCountdown(alert) {
+    const block = meetingBlocks.value.find(b => b.id === alert.blockId)
+    if (!block) return ''
+    const secs = secsUntil(block)
+    if (secs === null || secs <= 0) return 'Starting now!'
+    const m = Math.floor(secs / 60)
+    const s = secs % 60
+    return `${m}:${String(s).padStart(2, '0')}`
+  }
+
+  // Live TV auto-dismiss countdown
+  const tvCountdown = computed(() => {
+    if (!tvAlert.value) return 0
+    return Math.max(0, Math.round((tvAlert.value.autoDismissAt - now.value) / 1000))
+  })
+
+  function dismissDashboardAlert(id) {
+    dashboardAlerts.value = dashboardAlerts.value.filter(a => a.id !== id)
+  }
+
+  function dismissTvAlert() {
+    tvAlert.value = null
+  }
+
+  const interval = setInterval(() => {
+    now.value = new Date()
+
+    // Auto-dismiss TV alert when countdown expires
+    if (tvAlert.value && now.value >= tvAlert.value.autoDismissAt) {
+      tvAlert.value = null
+    }
+
+    meetingBlocks.value.forEach(block => {
+      const secs = secsUntil(block)
+      if (secs === null) return
+
+      const key5 = `${block.id}-5min`
+      const key1 = `${block.id}-1min`
+      const keyStart = `${block.id}-start`
+
+      // 5-minute warning
+      if (secs <= 300 && secs > 0 && !triggered.has(key5)) {
+        triggered.add(key5)
+        playChime(true)
+        dashboardAlerts.value.push({ id: key5, blockId: block.id, label: blockLabel(block) })
+      }
+
+      // 1-minute re-notify (only if user already dismissed the 5-min toast)
+      if (secs <= 60 && secs > 0 && !triggered.has(key1)) {
+        triggered.add(key1)
+        const stillActive = dashboardAlerts.value.some(a => a.blockId === block.id)
+        if (!stillActive) {
+          playChime(true)
+          dashboardAlerts.value.push({ id: key1, blockId: block.id, label: blockLabel(block) })
+        }
+      }
+
+      // At start time: full-screen TV alert, clear dashboard toasts
+      if (secs <= 0 && secs >= -15 && !triggered.has(keyStart)) {
+        triggered.add(keyStart)
+        dashboardAlerts.value = dashboardAlerts.value.filter(a => a.blockId !== block.id)
+        playChime(false)
+        if (onMeetingStart) onMeetingStart(block.id)
+        tvAlert.value = {
+          blockId: block.id,
+          label: blockLabel(block),
+          autoDismissAt: new Date(Date.now() + 30000),
+        }
+      }
+    })
+  }, 1000)
+
+  onUnmounted(() => clearInterval(interval))
+
+  return {
+    dashboardAlerts,
+    tvAlert,
+    tvCountdown,
+    alertCountdown,
+    dismissDashboardAlert,
+    dismissTvAlert,
+  }
+}