feat: add user display profiles and fix web context menu UX

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

app/users/router.py

@@ -55,7 +55,9 @@ async def update_profile(
     updated = await update_user_profile(
         db,
         current_user,
+        name=payload.name,
         username=payload.username,
+        bio=payload.bio,
         avatar_url=payload.avatar_url,
     )
     return updated