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',

src/store/auth.ts

@@ -359,12 +359,14 @@ export const useAuthStore = create<AuthState>()(
 
       registerWithEmail: async (email, password, firstName, referralCode) => {
         const code = referralCode || getPendingReferralCode() || undefined;
+        const campaignSlug = getPendingCampaignSlug() || undefined;
         const response = await authApi.registerEmailStandalone({
           email,
           password,
           first_name: firstName,
           language: navigator.language.split('-')[0] || 'ru',
           referral_code: code,
+          campaign_slug: campaignSlug,
         });
         consumeReferralCode();
         return response;