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

alembic/versions/0006_user_name_bio_profile.py

@@ -0,0 +1,29 @@
+"""add user name and bio
+
+Revision ID: 0006_user_name_bio_profile
+Revises: 0005_chat_public_saved_features
+Create Date: 2026-03-08 02:20:00.000000
+"""
+
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+revision: str = "0006_user_name_bio_profile"
+down_revision: Union[str, Sequence[str], None] = "0005_chat_public_saved_features"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column("users", sa.Column("name", sa.String(length=100), nullable=True))
+    op.add_column("users", sa.Column("bio", sa.String(length=500), nullable=True))
+    op.execute("UPDATE users SET name = username WHERE name IS NULL OR name = ''")
+    op.alter_column("users", "name", existing_type=sa.String(length=100), nullable=False)
+
+
+def downgrade() -> None:
+    op.drop_column("users", "bio")
+    op.drop_column("users", "name")