Add notification delivery model

[kibermaks]

Summary

This is PR 1 in the notification stack.

• Adds a typed notification delivery model for app events.
• Routes existing local notification posting through event-specific delivery settings.
• Keeps this layer isolated to model/local notification behavior: no preferences UI, no Apple Shortcuts, no webhooks, no quota migration.

Stack

1. notification-model - this PR, based on main
2. notification-shortcuts - local branch, based on notification-model
3. notification-webhooks - local branch, based on notification-shortcuts
4. quota-notifications-migration - local branch, based on notification-webhooks

The later branches are prepared locally but intentionally not opened yet.

Validation

• swift test --filter AppNotificationsTests on notification-model
• swift test --filter NotificationSettingsTests on full stack
• swift test --filter UsageStoreSessionQuotaTransitionTests on full stack
• swift test --filter PreferencesPaneSmokeTests on full stack

pnpm check is not applicable in this repo checkout because there is no package.json in the workspace root.