Notifications settings — macOS-style per-source controls

[jaylfc]

Spec: `docs/superpowers/specs/2026-04-14-settings-notifications-design.md`.

Mirror macOS Notifications preferences in the Settings app:

• List of sources (system.update, worker.join, worker.offline, agent.error, …)
• Per-source toggles: allow, style (none/banner/alert), show preview, play sound
• Global Do Not Disturb with schedule
• Persist via `/api/preferences/notifications`

Backend already has `NotificationStore.emit_event` — add a filter layer that consults prefs before inserting.