Add user profile management: display name + avatar upload (Gitea issue #12)

Users can set a display name (shown instead of username in the message
list, room member list, TopBar, and admin Users tab) and upload a real
avatar, replacing the generated color-initial avatars everywhere a user
appears. Avatars are square-cropped and downscaled to 512px, reusing
app/storage.py's upload primitives from image uploads with a new square
option.

Two deliberate divergences from message-image handling, documented in
backend/README.md: the previous avatar file is deleted on replace/remove
(safe since it's strictly one file per user), and avatar serving is not
room-gated and uses a short cache (identity-addressed and mutable, unlike
a message image's permanent content-addressed URL).

Frontend: new ProfileModal reachable from the TopBar account menu;
AuthContext gains updateUser() so a profile change reflects instantly
everywhere without a refetch.

backend/alembic/versions/f6e024985d4d_user_profile_fields.py

@@ -0,0 +1,36 @@
+"""user profile fields
+
+Revision ID: f6e024985d4d
+Revises: 1d355add7299
+Create Date: 2026-08-14 16:47:04.360957
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = 'f6e024985d4d'
+down_revision: Union[str, Sequence[str], None] = '1d355add7299'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    """Upgrade schema."""
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.add_column('users', sa.Column('display_name', sa.String(length=50), nullable=True))
+    op.add_column('users', sa.Column('avatar_filename', sa.String(length=64), nullable=True))
+    op.add_column('users', sa.Column('avatar_content_type', sa.String(length=50), nullable=True))
+    # ### end Alembic commands ###
+
+
+def downgrade() -> None:
+    """Downgrade schema."""
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.drop_column('users', 'avatar_content_type')
+    op.drop_column('users', 'avatar_filename')
+    op.drop_column('users', 'display_name')
+    # ### end Alembic commands ###