feat: send campaign_slug during standalone email registration

Frontend now reads campaign slug from localStorage and includes it
in the registration API call, ensuring campaign attribution works
even if email verification happens in a different browser.

src/api/auth.ts

@@ -97,6 +97,7 @@ export const authApi = {
     first_name?: string;
     language?: string;
     referral_code?: string;
+    campaign_slug?: string;
   }): Promise<RegisterResponse> => {
     const response = await apiClient.post<RegisterResponse>(
       '/cabinet/auth/email/register/standalone',