fix notification release modal flow

[JohnYin-hub]

Made-with: Cursor

Summary

• Reuse the existing /api/update/check result to build version update notifications, avoiding extra check_update() calls from the notification service.
• Combine benefit notifications and release-note notifications into one synchronized modal, preventing one section from appearing before the other is ready.
• Add localized release notes display by extracting Chinese or English sections from the same release body.
• Fix notification modal close behavior and preserve “don’t remind me again” acknowledgement for version notifications.

Test Plan

• source .venv/bin/activate && pytest tests/server/routes/test_notifications_routes.py tests/updater/test_updater.py::test_fetch_gitee_release_returns_web_archive_urls
• npm run lint
• npm run test:run -- src/components/layout/Layout.test.tsx
• npm run test:run -- src/components/layout/Layout.test.tsx src/utils/releaseNotes.test.ts

Notes

• check_update() core logic is intentionally unchanged to avoid impacting update/apply and CLI upgrade flows.
• Release notes localization expects the release body to contain language sections such as ## 中文 and ## English; if no matching section is found, the full release notes are shown.