refactor(auth): remove username fallback

src/data/repositories/auth_repository.ts

@@ -76,7 +76,7 @@ export class AuthRepository implements IAuthRepository {
         LoginRequest.from(
           withTestAccountFlag({
             email,
-            username: resolveLoginUsername(input.username, email),
+            username: input.username,
             password: input.password,
             platform: getAuthPlatform(),
             guestId: input.guestId ?? "",
@@ -341,11 +341,3 @@ function withTestAccountFlag<T extends Record<string, unknown>>(
 function getAuthPlatform(): string {
   return PlatformDetector.getPlatform();
 }
-
-function resolveLoginUsername(
-  username: string | undefined,
-  email: string,
-): string {
-  const trimmedUsername = username?.trim() ?? "";
-  return trimmedUsername.length > 0 ? trimmedUsername : email;
-}