feat: update customer-facing strings from 'login'/'logout' to 'sign in'/'sign out' #20891
Description
What would you like to be added?
Update all customer-facing strings, including CLI command names, UI labels, and documentation, to use 'sign in' and 'sign out' instead of 'login' and 'logout' for Google accounts. This includes renaming the /login command to /signin and /logout to /signout.
Why is this needed?
Google-branded account authentication should use 'sign in' and 'sign out' terminology to align with Google's design and branding standards. This improves clarity for users and ensures consistency with other Google services.
Additional context
This change involves updates to:
- Slash commands: /login -> /signin, /logout -> /signout
- UI labels: 'Login with Google' -> 'Sign in with Google', 'Logged in' -> 'Signed in'
- Documentation: README.md and all docs in docs/
- All related tests.