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/app/models/user.py

@@ -17,6 +17,9 @@ class User(Base):
     is_bot: Mapped[bool] = mapped_column(Boolean, default=False, nullable=False)
     is_site_admin: Mapped[bool] = mapped_column(Boolean, default=False, nullable=False)
     is_active: Mapped[bool] = mapped_column(Boolean, default=True, nullable=False)
+    display_name: Mapped[str | None] = mapped_column(String(50))
+    avatar_filename: Mapped[str | None] = mapped_column(String(64))
+    avatar_content_type: Mapped[str | None] = mapped_column(String(50))
     created_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True), server_default=func.now(), nullable=False
     )