Add push notification functionality with tests and digest scheduler

- Implemented push subscription API with tests for subscribing and unsubscribing users.
- Created web push notification tests triggered by child actions.
- Added digest scheduler to send email digests to users at 9 PM local time.
- Developed utility functions for creating and validating digest action tokens.
- Integrated web push sender to handle sending notifications to users.
- Added service worker for handling push notifications in the frontend.
- Created a push opt-in component for user notification preferences.
- Implemented tests for the push opt-in component to ensure correct behavior.
- Updated frontend services to manage push subscriptions and permissions.

frontend/vue-app/src/services/pushSubscription.ts

@@ -0,0 +1,103 @@
+/**
+ * Push subscription manager.
+ * Handles requesting notification permission, subscribing to push,
+ * and syncing the subscription with the backend.
+ */
+
+async function fetchVapidPublicKey(): Promise<string | null> {
+  try {
+    const res = await fetch('/api/push-vapid-key')
+    if (!res.ok) return null
+    const data = await res.json()
+    return data.public_key || null
+  } catch {
+    return null
+  }
+}
+
+function urlBase64ToUint8Array(base64String: string): Uint8Array {
+  const padding = '='.repeat((4 - (base64String.length % 4)) % 4)
+  const base64 = (base64String + padding).replace(/-/g, '+').replace(/_/g, '/')
+  const rawData = window.atob(base64)
+  return Uint8Array.from([...rawData].map((c) => c.charCodeAt(0)))
+}
+
+/**
+ * Request notification permission and subscribe to push notifications.
+ * Posts the subscription to the backend, including the user's timezone.
+ * Returns true if subscription was successfully registered.
+ */
+export async function subscribeToPush(): Promise<boolean> {
+  if (!('serviceWorker' in navigator) || !('PushManager' in window)) {
+    return false
+  }
+
+  try {
+    const permission = await Notification.requestPermission()
+    if (permission !== 'granted') {
+      return false
+    }
+
+    const vapidKey = await fetchVapidPublicKey()
+    if (!vapidKey) {
+      console.warn('[Push] VAPID public key not available; skipping subscription')
+      return false
+    }
+
+    const registration = await navigator.serviceWorker.ready
+    const subscription = await registration.pushManager.subscribe({
+      userVisibleOnly: true,
+      applicationServerKey: urlBase64ToUint8Array(vapidKey),
+    })
+
+    const subJson = subscription.toJSON()
+    const timezone = Intl.DateTimeFormat().resolvedOptions().timeZone
+
+    const res = await fetch('/api/push-subscription', {
+      method: 'POST',
+      headers: { 'Content-Type': 'application/json' },
+      body: JSON.stringify({
+        endpoint: subJson.endpoint,
+        keys: subJson.keys,
+        timezone,
+      }),
+    })
+
+    return res.ok
+  } catch (err) {
+    console.warn('[Push] Failed to subscribe:', err)
+    return false
+  }
+}
+
+/**
+ * Unsubscribe from push notifications and remove the subscription from the backend.
+ */
+export async function unsubscribeFromPush(): Promise<void> {
+  if (!('serviceWorker' in navigator) || !('PushManager' in window)) return
+
+  try {
+    const registration = await navigator.serviceWorker.ready
+    const subscription = await registration.pushManager.getSubscription()
+    if (!subscription) return
+
+    const endpoint = subscription.endpoint
+    await subscription.unsubscribe()
+
+    await fetch('/api/push-subscription', {
+      method: 'DELETE',
+      headers: { 'Content-Type': 'application/json' },
+      body: JSON.stringify({ endpoint }),
+    })
+  } catch (err) {
+    console.warn('[Push] Failed to unsubscribe:', err)
+  }
+}
+
+/**
+ * Check the current push permission state without prompting.
+ */
+export function getPushPermissionState(): NotificationPermission | 'unsupported' {
+  if (!('Notification' in window)) return 'unsupported'
+  return Notification.permission
+}