Adds a native OS notification system (Electron Notification, routed cross-OS)
distinct from the in-app toast feed. Before this, one hardcoded cue existed
(message.complete while document.hidden) with no settings or event coverage.
- Engine (store/native-notifications.ts): localStorage-backed prefs (master
switch + per-kind toggles) and a gated dispatcher over five kinds — approval,
input, turnDone, turnError, backgroundDone — with a 1s per-(kind,session)
self-evicting throttle.
- Gating: "backgrounded" = document.hidden OR !document.hasFocus(), so an
alt-tabbed window still counts as away. Completion kinds fire only when
backgrounded and for the active session (no spam from a busy gateway);
attention kinds (approval/input) also break through for off-screen sessions.
- Wired into real event sites (use-message-stream.ts): message.complete, error,
approval/clarify/sudo/secret.request; backgroundDone from composer-status at
the running -> exited transition.
- Click focuses the window and jumps to the originating session; approval
notifications carry Approve/Reject buttons that resolve in place over
approval.respond, mirroring the in-app Run/Reject bar.
- Settings: new Notifications panel (master + per-kind switches, test button
with real OS-result feedback). Full i18n (en/ja/zh/zh-hant).
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