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/auth/service.py

@@ -54,6 +54,7 @@ async def register_user(
     user = await create_user(
         db,
         email=payload.email,
+        name=payload.name,
         username=payload.username,
         password_hash=hash_password(payload.password),
     )