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/UserAvatar.tsx

@@ -5,9 +5,21 @@ interface UserAvatarProps {
   username: string
   colorIndex: number
   size?: number
+  avatarUrl?: string | null
 }
 
-export function UserAvatar({ username, colorIndex, size = 28 }: UserAvatarProps) {
+export function UserAvatar({ username, colorIndex, size = 28, avatarUrl }: UserAvatarProps) {
+  if (avatarUrl) {
+    return (
+      <img
+        src={avatarUrl}
+        alt=""
+        className="user-avatar user-avatar-img"
+        style={{ width: size, height: size }}
+      />
+    )
+  }
+
   return (
     <div
       className="user-avatar"