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/main.py

@@ -11,7 +11,7 @@ from redis.asyncio import Redis
 from starlette.middleware.sessions import SessionMiddleware
 
 from app.config import settings
-from app.routers import admin, auth, bots, health, invites, push, rooms, webhooks
+from app.routers import admin, auth, bots, health, invites, push, rooms, users, webhooks
 from app.ws.broadcaster import RoomBroadcaster
 from app.ws.chat import router as ws_router
 from app.ws.connection_manager import ConnectionManager
@@ -73,6 +73,7 @@ def create_app() -> FastAPI:
     app.include_router(health.router)
     app.include_router(auth.router)
     app.include_router(rooms.router)
+    app.include_router(users.router)
     app.include_router(invites.router)
     app.include_router(push.router)
     app.include_router(admin.router)