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/context/AuthContext.tsx

@@ -10,6 +10,7 @@ interface AuthContextValue {
   offline: boolean
   login: (usernameOrEmail: string, password: string) => Promise<void>
   logout: () => Promise<void>
+  updateUser: (user: User) => void
 }
 
 const AuthContext = createContext<AuthContextValue | undefined>(undefined)
@@ -67,8 +68,13 @@ export function AuthProvider({ children }: { children: ReactNode }) {
     clearLastUser()
   }
 
+  function updateUser(u: User) {
+    setUser(u)
+    saveLastUser(u)
+  }
+
   return (
-    <AuthContext.Provider value={{ user, loading, offline, login, logout }}>
+    <AuthContext.Provider value={{ user, loading, offline, login, logout, updateUser }}>
       {children}
     </AuthContext.Provider>
   )