feat: add mode parameter to social auth to distinguish between login and signup flows

src/auth/auth.service.ts

@@ -256,8 +256,13 @@ export class AuthService {
           where: { id: user.id },
           data: { [providerIdField]: dto.providerId },
         });
+      } else if (dto.mode === 'login') {
+        // Login mode: user must exist
+        throw new BadRequestException(
+          'No account found with this email. Please sign up first.',
+        );
       } else {
-        // Create new user
+        // Create new user (signup mode)
         const role = dto.role === 'AGENT' ? UserRole.AGENT : UserRole.USER;
         const authProvider =
           dto.provider === 'google'