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/chats/repository.py

@@ -159,3 +159,9 @@ async def find_private_chat_between_users(db: AsyncSession, *, user_a_id: int, u
     )
     result = await db.execute(stmt)
     return result.scalar_one_or_none()
+
+
+async def get_private_counterpart_user_id(db: AsyncSession, *, chat_id: int, user_id: int) -> int | None:
+    stmt = select(ChatMember.user_id).where(ChatMember.chat_id == chat_id, ChatMember.user_id != user_id).limit(1)
+    result = await db.execute(stmt)
+    return result.scalar_one_or_none()