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/models.py

@@ -16,10 +16,12 @@ class User(Base):
     __tablename__ = "users"
 
     id: Mapped[int] = mapped_column(primary_key=True, index=True)
+    name: Mapped[str] = mapped_column(String(100), nullable=False)
     username: Mapped[str] = mapped_column(String(50), unique=True, index=True)
     email: Mapped[str] = mapped_column(String(255), unique=True, index=True)
     password_hash: Mapped[str] = mapped_column(String(255))
     avatar_url: Mapped[str | None] = mapped_column(String(512), nullable=True)
+    bio: Mapped[str | None] = mapped_column(String(500), nullable=True)
     email_verified: Mapped[bool] = mapped_column(Boolean, default=False, nullable=False, index=True)
     created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), server_default=func.now(), nullable=False)
     updated_at: Mapped[datetime] = mapped_column(