Message Anchor is now the default navigation bar

What changed

The message navigation bar now defaults to Message Anchor instead of None.

Why this matters to the user

New users and v1 users who never selected a message navigation mode will see the message anchor rail beside conversations. Existing saved selections remain unchanged.

What the user should do

Nothing — automatic. Users can still choose None or Navigation Buttons in Message Settings.

Notes for release manager

This changes only the fallback for a missing preference. It does not overwrite an existing chat.message.navigation_mode value.