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/service.py

@@ -36,11 +36,17 @@ async def update_user_profile(
     db: AsyncSession,
     user: User,
     *,
+    name: str | None = None,
     username: str | None = None,
+    bio: str | None = None,
     avatar_url: str | None = None,
 ) -> User:
+    if name is not None:
+        user.name = name
     if username is not None:
         user.username = username
+    if bio is not None:
+        user.bio = bio
     if avatar_url is not None:
         user.avatar_url = avatar_url
     await db.commit()