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.

frontend/src/components/TopBar.css

@@ -40,16 +40,11 @@
 .top-bar-avatar {
   width: 30px;
   height: 30px;
-  border-radius: var(--radius-pill);
-  background: var(--ds-accent-3);
+  padding: 0;
   border: none;
-  color: var(--ds-text);
-  font-size: 0.78rem;
-  font-weight: 700;
-  display: flex;
-  align-items: center;
-  justify-content: center;
+  background: transparent;
   cursor: pointer;
+  display: flex;
 }
 
 .top-bar-menu-scrim {