feat: add system settings page for rollup retention

[hardy-dev-infinilabs]

Summary

• add backend APIs and tests for system retention and rollup settings
• add a /system/settings page with general and email tabs
• route /system/email_server through the new settings page and embed the email server list

Testing

• go test ./plugin/api/settings ./plugin/api
• NODE_OPTIONS='--openssl-legacy-provider --max_old_space_size=4096' ./node_modules/.bin/umi build