Desktop notifications: UNUserNotificationCenter (post/update/remove, actions) #26

What

No way to post a user notification (banner/alert). The current and correct
API is the UserNotifications framework — UNUserNotificationCenter,
which replaced the deprecated NSUserNotification / NSUserNotificationCenter
(deprecated since macOS 11, unreliable now). This is the cocoa counterpart of
freedesktop org.freedesktop.Notifications (react-x11#353).

The bundle-identity constraint (the main gotcha)

UNUserNotificationCenter.current() delivers nothing — and can throw — unless
the process is a code-signed .app bundle with a real CFBundleIdentifier.
The cocoa backend runs today as a bare node process whose CFBundleIdentifier
is null, so UN… posts silently do nothing until the app is packaged. The
bridge should make this a capability the renderer can read, not a silent
drop — e.g. a notificationsAvailable() / settings query that reflects whether
a bundle id resolves — so the consumer can fall to its osascript rung
instead.

Proposed API (mechanism only; policy stays in the renderer)

native.requestNotificationAuthorization(['alert','sound','badge'], cb)  // cb(granted)
native.notificationSettings(cb)   // authorization status + which are enabled + bundle-id present

const id = native.postNotification({
  identifier?,          // stable id; reusing one REPLACES the banner (the replaces_id case)
  title, subtitle?, body?,
  sound?: 'default' | null,
  categoryId?,          // ties to a registered action set
  userInfo?,            // opaque, round-tripped back on action / dismiss
})
native.updateNotification(id, { … })   // = post with the same identifier
native.removeNotification(id)          // removeDeliveredNotificationsWithIdentifiers:
native.setNotificationCategories([
  { id, actions: [{ id, title, destructive?, foreground? }] },
])
// backend events:
//   notification-action     { identifier, actionId, userInfo }
//   notification-dismissed   { identifier, reason }

• Set a UNUserNotificationCenterDelegate at startup so a banner acted on
  while the app is running routes back (didReceive), and so willPresent
  can show banners while the app is foregrounded.
• NSUserNotification is prior art but deprecated — target UN….

Consumer

react-x11 notifications, the macOS half of react-x11#353. See docs/macos.md.