backend:
- add required user name and optional bio fields
- extend auth/register and user schemas/services with name/bio
- add alembic migration 0006 with safe backfill name=username
- compute per-user chat display_title for private chats
- keep Saved Messages delete-for-all protections
web:
- registration now includes name
- add profile edit modal (name/username/bio/avatar url)
- show private chat names via display_title
- fix context menus to open near cursor with viewport clamping
- stabilize +/close floating button to remove visual jump
