feat(push): owner enrolment UI, service worker and VAPID keygen (T10)

EnableNotifications subscribes the owner's device with the public VAPID
key and stores it; sw.js receives pushes and opens /defects on click;
`pnpm vapid:keys` mints the key pair.

app/components/EnableNotifications.vue

@@ -0,0 +1,116 @@
+<script setup lang="ts">
+// Owner-only Web Push enrolment (T10/F8). Renders nothing unless the signed-in
+// user is the configured owner. Registers the service worker, subscribes the
+// device with the public VAPID key, and stores the subscription server-side;
+// turning it off unsubscribes and prunes the row.
+const { user } = useUserSession()
+const config = useRuntimeConfig()
+const ownerEmail = config.public.ownerEmail
+const vapidPublicKey = config.public.vapidPublicKey
+
+const isOwner = computed(
+  () =>
+    !!user.value?.email &&
+    !!ownerEmail &&
+    user.value.email.toLowerCase() === ownerEmail.toLowerCase(),
+)
+
+type State = 'loading' | 'idle' | 'subscribed' | 'denied' | 'unsupported' | 'misconfigured'
+const state = ref<State>('loading')
+const busy = ref(false)
+
+const supported = () =>
+  import.meta.client &&
+  'serviceWorker' in navigator &&
+  'PushManager' in window &&
+  'Notification' in window
+
+onMounted(async () => {
+  if (!isOwner.value) return
+  if (!supported()) return void (state.value = 'unsupported')
+  if (!vapidPublicKey) return void (state.value = 'misconfigured')
+  if (Notification.permission === 'denied') return void (state.value = 'denied')
+
+  const reg = await navigator.serviceWorker.getRegistration()
+  const existing = await reg?.pushManager.getSubscription()
+  state.value = existing ? 'subscribed' : 'idle'
+})
+
+async function enable() {
+  busy.value = true
+  try {
+    const reg = await navigator.serviceWorker.register('/sw.js')
+    await navigator.serviceWorker.ready
+
+    const permission = await Notification.requestPermission()
+    if (permission !== 'granted') {
+      state.value = permission === 'denied' ? 'denied' : 'idle'
+      return
+    }
+
+    const sub = await reg.pushManager.subscribe({
+      userVisibleOnly: true,
+      applicationServerKey: urlBase64ToUint8Array(vapidPublicKey),
+    })
+    await $fetch('/api/subscriptions', { method: 'POST', body: sub.toJSON() })
+    state.value = 'subscribed'
+  } finally {
+    busy.value = false
+  }
+}
+
+async function disable() {
+  busy.value = true
+  try {
+    const reg = await navigator.serviceWorker.getRegistration()
+    const sub = await reg?.pushManager.getSubscription()
+    if (sub) {
+      await $fetch('/api/subscriptions', { method: 'DELETE', body: { endpoint: sub.endpoint } })
+      await sub.unsubscribe()
+    }
+    state.value = 'idle'
+  } finally {
+    busy.value = false
+  }
+}
+
+// VAPID public keys are URL-safe base64; PushManager wants the raw bytes,
+// backed by a concrete ArrayBuffer to satisfy the BufferSource type.
+function urlBase64ToUint8Array(base64: string): Uint8Array<ArrayBuffer> {
+  const padding = '='.repeat((4 - (base64.length % 4)) % 4)
+  const normalised = (base64 + padding).replace(/-/g, '+').replace(/_/g, '/')
+  const raw = atob(normalised)
+  const bytes = new Uint8Array(new ArrayBuffer(raw.length))
+  for (let i = 0; i < raw.length; i++) bytes[i] = raw.charCodeAt(i)
+  return bytes
+}
+</script>
+
+<template>
+  <div v-if="isOwner" class="text-sm">
+    <div v-if="state === 'subscribed'" class="flex items-center gap-2">
+      <span class="opacity-70">🔔 Notifications on</span>
+      <button type="button" class="btn btn-ghost btn-xs" :disabled="busy" @click="disable">
+        Turn off
+      </button>
+    </div>
+    <button
+      v-else-if="state === 'idle'"
+      type="button"
+      class="btn btn-outline btn-sm"
+      :disabled="busy"
+      @click="enable"
+    >
+      Enable notifications
+    </button>
+    <p v-else-if="state === 'denied'" class="opacity-60">
+      Notifications are blocked in your browser settings.
+    </p>
+    <p v-else-if="state === 'unsupported'" class="opacity-60">
+      This browser can’t receive push notifications.
+    </p>
+    <p v-else-if="state === 'misconfigured'" class="opacity-60">
+      Push notifications aren’t configured.
+    </p>
+  </div>
+</template>