feat(webui): push change notices to open pages over SSE

[Col0ring]

Summary

Server-sent change stream

• add an in-process event bus and a GET /api/events SSE endpoint that fans a change notice out to every open page
• publish a notice from the envelope route after a successful management write (skipping presence, chat, recovery and the event stream itself), so a mutation made by any caller — including an external script or another tab — refreshes every open page
• open one EventSource per tab at the app shell and revalidate the active routes on a change notice, coalescing a burst into one refresh; replay the matching in-tab event so components that hold their own state react to an external change like a local one
• add a models-changed event and re-fetch the model catalog in the composer picker and the models settings page when it fires

Delete flow

• when deleting the open project or session, await the navigation to a live URL before revalidating, so the refresh no longer aborts the route switch or flashes the deleted route's 404

Test

• pnpm typecheck
• pytest tests/test_state_lifecycle_api.py tests/test_chat.py (111 passed)